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.
(16 + -1x)(25 + x) = 0

Subproblem 1
Set the factor '(16 + -1x)' equal to zero and attempt to solve:

Simplifying
16 + -1x = 0

Solving
16 + -1x = 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 + -1x = 0 + -16

Combine like terms: 16 + -16 = 0
0 + -1x = 0 + -16
-1x = 0 + -16

Combine like terms: 0 + -16 = -16
-1x = -16

Divide each side by '-1'.
x = 16

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

Simplifying
25 + x = 0

Solving
25 + x = 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 + x = 0 + -25

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

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

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