8=(x+2)(2x+1)

Solution for 8=(x+2)(2x+1) equation:

Simplifying
8 = (x + 2)(2x + 1)

Reorder the terms:
8 = (2 + x)(2x + 1)

Reorder the terms:
8 = (2 + x)(1 + 2x)

Multiply (2 + x) * (1 + 2x)
8 = (2(1 + 2x) + x(1 + 2x))
8 = ((1 * 2 + 2x * 2) + x(1 + 2x))
8 = ((2 + 4x) + x(1 + 2x))
8 = (2 + 4x + (1 * x + 2x * x))
8 = (2 + 4x + (1x + 2x2))

Combine like terms: 4x + 1x = 5x
8 = (2 + 5x + 2x2)

Solving
8 = 2 + 5x + 2x2

Solving for variable 'x'.

Combine like terms: 8 + -2 = 6
6 + -5x + -2x2 = 2 + 5x + 2x2 + -2 + -5x + -2x2

Reorder the terms:
6 + -5x + -2x2 = 2 + -2 + 5x + -5x + 2x2 + -2x2

Combine like terms: 2 + -2 = 0
6 + -5x + -2x2 = 0 + 5x + -5x + 2x2 + -2x2
6 + -5x + -2x2 = 5x + -5x + 2x2 + -2x2

Combine like terms: 5x + -5x = 0
6 + -5x + -2x2 = 0 + 2x2 + -2x2
6 + -5x + -2x2 = 2x2 + -2x2

Combine like terms: 2x2 + -2x2 = 0
6 + -5x + -2x2 = 0

Begin completing the square.  Divide all terms by
-2 the coefficient of the squared term: 

Divide each side by '-2'.
-3 + 2.5x + x2 = 0

Move the constant term to the right:

Add '3' to each side of the equation.
-3 + 2.5x + 3 + x2 = 0 + 3

Reorder the terms:
-3 + 3 + 2.5x + x2 = 0 + 3

Combine like terms: -3 + 3 = 0
0 + 2.5x + x2 = 0 + 3
2.5x + x2 = 0 + 3

Combine like terms: 0 + 3 = 3
2.5x + x2 = 3

The x term is 2.5x.  Take half its coefficient (1.25).
Square it (1.5625) and add it to both sides.

Add '1.5625' to each side of the equation.
2.5x + 1.5625 + x2 = 3 + 1.5625

Reorder the terms:
1.5625 + 2.5x + x2 = 3 + 1.5625

Combine like terms: 3 + 1.5625 = 4.5625
1.5625 + 2.5x + x2 = 4.5625

Factor a perfect square on the left side:
(x + 1.25)(x + 1.25) = 4.5625

Calculate the square root of the right side: 2.136000936

Break this problem into two subproblems by setting 
(x + 1.25) equal to 2.136000936 and -2.136000936.

Subproblem 1
x + 1.25 = 2.136000936

Simplifying
x + 1.25 = 2.136000936

Reorder the terms:
1.25 + x = 2.136000936

Solving
1.25 + x = 2.136000936

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Add '-1.25' to each side of the equation.
1.25 + -1.25 + x = 2.136000936 + -1.25

Combine like terms: 1.25 + -1.25 = 0.00
0.00 + x = 2.136000936 + -1.25
x = 2.136000936 + -1.25

Combine like terms: 2.136000936 + -1.25 = 0.886000936
x = 0.886000936

Simplifying
x = 0.886000936

Subproblem 2
x + 1.25 = -2.136000936

Simplifying
x + 1.25 = -2.136000936

Reorder the terms:
1.25 + x = -2.136000936

Solving
1.25 + x = -2.136000936

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Add '-1.25' to each side of the equation.
1.25 + -1.25 + x = -2.136000936 + -1.25

Combine like terms: 1.25 + -1.25 = 0.00
0.00 + x = -2.136000936 + -1.25
x = -2.136000936 + -1.25

Combine like terms: -2.136000936 + -1.25 = -3.386000936
x = -3.386000936

Simplifying
x = -3.386000936

Solution
The solution to the problem is based on the solutions
from the subproblems.
x = {0.886000936, -3.386000936}