3x(x+7)=4x+21

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Solution for 3x(x+7)=4x+21 equation:


Simplifying
3x(x + 7) = 4x + 21

Reorder the terms:
3x(7 + x) = 4x + 21
(7 * 3x + x * 3x) = 4x + 21
(21x + 3x2) = 4x + 21

Reorder the terms:
21x + 3x2 = 21 + 4x

Solving
21x + 3x2 = 21 + 4x

Solving for variable 'x'.

Reorder the terms:
-21 + 21x + -4x + 3x2 = 21 + 4x + -21 + -4x

Combine like terms: 21x + -4x = 17x
-21 + 17x + 3x2 = 21 + 4x + -21 + -4x

Reorder the terms:
-21 + 17x + 3x2 = 21 + -21 + 4x + -4x

Combine like terms: 21 + -21 = 0
-21 + 17x + 3x2 = 0 + 4x + -4x
-21 + 17x + 3x2 = 4x + -4x

Combine like terms: 4x + -4x = 0
-21 + 17x + 3x2 = 0

Begin completing the square.  Divide all terms by
3 the coefficient of the squared term: 

Divide each side by '3'.
-7 + 5.666666667x + x2 = 0

Move the constant term to the right:

Add '7' to each side of the equation.
-7 + 5.666666667x + 7 + x2 = 0 + 7

Reorder the terms:
-7 + 7 + 5.666666667x + x2 = 0 + 7

Combine like terms: -7 + 7 = 0
0 + 5.666666667x + x2 = 0 + 7
5.666666667x + x2 = 0 + 7

Combine like terms: 0 + 7 = 7
5.666666667x + x2 = 7

The x term is 5.666666667x.  Take half its coefficient (2.833333334).
Square it (8.027777782) and add it to both sides.

Add '8.027777782' to each side of the equation.
5.666666667x + 8.027777782 + x2 = 7 + 8.027777782

Reorder the terms:
8.027777782 + 5.666666667x + x2 = 7 + 8.027777782

Combine like terms: 7 + 8.027777782 = 15.027777782
8.027777782 + 5.666666667x + x2 = 15.027777782

Factor a perfect square on the left side:
(x + 2.833333334)(x + 2.833333334) = 15.027777782

Calculate the square root of the right side: 3.876567784

Break this problem into two subproblems by setting 
(x + 2.833333334) equal to 3.876567784 and -3.876567784.

Subproblem 1

x + 2.833333334 = 3.876567784 Simplifying x + 2.833333334 = 3.876567784 Reorder the terms: 2.833333334 + x = 3.876567784 Solving 2.833333334 + x = 3.876567784 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-2.833333334' to each side of the equation. 2.833333334 + -2.833333334 + x = 3.876567784 + -2.833333334 Combine like terms: 2.833333334 + -2.833333334 = 0.000000000 0.000000000 + x = 3.876567784 + -2.833333334 x = 3.876567784 + -2.833333334 Combine like terms: 3.876567784 + -2.833333334 = 1.04323445 x = 1.04323445 Simplifying x = 1.04323445

Subproblem 2

x + 2.833333334 = -3.876567784 Simplifying x + 2.833333334 = -3.876567784 Reorder the terms: 2.833333334 + x = -3.876567784 Solving 2.833333334 + x = -3.876567784 Solving for variable 'x'. Move all terms containing x to the left, all other terms to the right. Add '-2.833333334' to each side of the equation. 2.833333334 + -2.833333334 + x = -3.876567784 + -2.833333334 Combine like terms: 2.833333334 + -2.833333334 = 0.000000000 0.000000000 + x = -3.876567784 + -2.833333334 x = -3.876567784 + -2.833333334 Combine like terms: -3.876567784 + -2.833333334 = -6.709901118 x = -6.709901118 Simplifying x = -6.709901118

Solution

The solution to the problem is based on the solutions from the subproblems. x = {1.04323445, -6.709901118}

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