21=3.5x*x

Solution for 21=3.5x*x equation:

Simplifying
21 = 3.5x * x

Multiply x * x
21 = 3.5x2

Solving
21 = 3.5x2

Solving for variable 'x'.

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

Add '-3.5x2' to each side of the equation.
21 + -3.5x2 = 3.5x2 + -3.5x2

Combine like terms: 3.5x2 + -3.5x2 = 0.0
21 + -3.5x2 = 0.0

Add '-21' to each side of the equation.
21 + -21 + -3.5x2 = 0.0 + -21

Combine like terms: 21 + -21 = 0
0 + -3.5x2 = 0.0 + -21
-3.5x2 = 0.0 + -21

Combine like terms: 0.0 + -21 = -21
-3.5x2 = -21

Divide each side by '-3.5'.
x2 = 6

Simplifying
x2 = 6

Take the square root of each side:
x = {-2.449489743, 2.449489743}