400=x(x-9)

Solution for 400=x(x-9) equation:

Simplifying
400 = x(x + -9)

Reorder the terms:
400 = x(-9 + x)
400 = (-9 * x + x * x)
400 = (-9x + x2)

Solving
400 = -9x + x2

Solving for variable 'x'.

Reorder the terms:
400 + 9x + -1x2 = -9x + 9x + x2 + -1x2

Combine like terms: -9x + 9x = 0
400 + 9x + -1x2 = 0 + x2 + -1x2
400 + 9x + -1x2 = x2 + -1x2

Combine like terms: x2 + -1x2 = 0
400 + 9x + -1x2 = 0

Factor a trinomial.
(25 + -1x)(16 + x) = 0

Subproblem 1
Set the factor '(25 + -1x)' equal to zero and attempt to solve:

Simplifying
25 + -1x = 0

Solving
25 + -1x = 0

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

Add '-25' to each side of the equation.
25 + -25 + -1x = 0 + -25

Combine like terms: 25 + -25 = 0
0 + -1x = 0 + -25
-1x = 0 + -25

Combine like terms: 0 + -25 = -25
-1x = -25

Divide each side by '-1'.
x = 25

Simplifying
x = 25
Subproblem 2
Set the factor '(16 + x)' equal to zero and attempt to solve:

Simplifying
16 + x = 0

Solving
16 + x = 0

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

Add '-16' to each side of the equation.
16 + -16 + x = 0 + -16

Combine like terms: 16 + -16 = 0
0 + x = 0 + -16
x = 0 + -16

Combine like terms: 0 + -16 = -16
x = -16

Simplifying
x = -16
Solution
x = {25, -16}