100=x(x+15)

Solution for 100=x(x+15) equation:

Simplifying
100 = x(x + 15)

Reorder the terms:
100 = x(15 + x)
100 = (15 * x + x * x)
100 = (15x + x2)

Solving
100 = 15x + x2

Solving for variable 'x'.

Reorder the terms:
100 + -15x + -1x2 = 15x + -15x + x2 + -1x2

Combine like terms: 15x + -15x = 0
100 + -15x + -1x2 = 0 + x2 + -1x2
100 + -15x + -1x2 = x2 + -1x2

Combine like terms: x2 + -1x2 = 0
100 + -15x + -1x2 = 0

Factor a trinomial.
(5 + -1x)(20 + x) = 0

Subproblem 1
Set the factor '(5 + -1x)' equal to zero and attempt to solve:

Simplifying
5 + -1x = 0

Solving
5 + -1x = 0

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

Add '-5' to each side of the equation.
5 + -5 + -1x = 0 + -5

Combine like terms: 5 + -5 = 0
0 + -1x = 0 + -5
-1x = 0 + -5

Combine like terms: 0 + -5 = -5
-1x = -5

Divide each side by '-1'.
x = 5

Simplifying
x = 5
Subproblem 2
Set the factor '(20 + x)' equal to zero and attempt to solve:

Simplifying
20 + x = 0

Solving
20 + x = 0

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

Add '-20' to each side of the equation.
20 + -20 + x = 0 + -20

Combine like terms: 20 + -20 = 0
0 + x = 0 + -20
x = 0 + -20

Combine like terms: 0 + -20 = -20
x = -20

Simplifying
x = -20
Solution
x = {5, -20}