90=x(x-8)

Solution for 90=x(x-8) equation:

Simplifying
90 = x(x + -8)

Reorder the terms:
90 = x(-8 + x)
90 = (-8 * x + x * x)
90 = (-8x + x2)

Solving
90 = -8x + x2

Solving for variable 'x'.

Reorder the terms:
90 + 8x + -1x2 = -8x + 8x + x2 + -1x2

Combine like terms: -8x + 8x = 0
90 + 8x + -1x2 = 0 + x2 + -1x2
90 + 8x + -1x2 = x2 + -1x2

Combine like terms: x2 + -1x2 = 0
90 + 8x + -1x2 = 0

Begin completing the square.  Divide all terms by
-1 the coefficient of the squared term: 

Divide each side by '-1'.
-90 + -8x + x2 = 0

Move the constant term to the right:

Add '90' to each side of the equation.
-90 + -8x + 90 + x2 = 0 + 90

Reorder the terms:
-90 + 90 + -8x + x2 = 0 + 90

Combine like terms: -90 + 90 = 0
0 + -8x + x2 = 0 + 90
-8x + x2 = 0 + 90

Combine like terms: 0 + 90 = 90
-8x + x2 = 90

The x term is -8x.  Take half its coefficient (-4).
Square it (16) and add it to both sides.

Add '16' to each side of the equation.
-8x + 16 + x2 = 90 + 16

Reorder the terms:
16 + -8x + x2 = 90 + 16

Combine like terms: 90 + 16 = 106
16 + -8x + x2 = 106

Factor a perfect square on the left side:
(x + -4)(x + -4) = 106

Calculate the square root of the right side: 10.295630141

Break this problem into two subproblems by setting 
(x + -4) equal to 10.295630141 and -10.295630141.

Subproblem 1
x + -4 = 10.295630141

Simplifying
x + -4 = 10.295630141

Reorder the terms:
-4 + x = 10.295630141

Solving
-4 + x = 10.295630141

Solving for variable 'x'.

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

Add '4' to each side of the equation.
-4 + 4 + x = 10.295630141 + 4

Combine like terms: -4 + 4 = 0
0 + x = 10.295630141 + 4
x = 10.295630141 + 4

Combine like terms: 10.295630141 + 4 = 14.295630141
x = 14.295630141

Simplifying
x = 14.295630141

Subproblem 2
x + -4 = -10.295630141

Simplifying
x + -4 = -10.295630141

Reorder the terms:
-4 + x = -10.295630141

Solving
-4 + x = -10.295630141

Solving for variable 'x'.

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

Add '4' to each side of the equation.
-4 + 4 + x = -10.295630141 + 4

Combine like terms: -4 + 4 = 0
0 + x = -10.295630141 + 4
x = -10.295630141 + 4

Combine like terms: -10.295630141 + 4 = -6.295630141
x = -6.295630141

Simplifying
x = -6.295630141

Solution
The solution to the problem is based on the solutions
from the subproblems.
x = {14.295630141, -6.295630141}