f(0)=x^2-21x

Solution for f(0)=x^2-21x equation:

Simplifying
f(0) = x2 + -21x

Reorder the terms for easier multiplication:
0f = x2 + -21x

Anything times zero is zero.
0f = x2 + -21x

Reorder the terms:
0 = -21x + x2

Solving
0 = -21x + x2

Solving for variable 'x'.
Remove the zero:
21x + -1x2 = -21x + x2 + 21x + -1x2

Reorder the terms:
21x + -1x2 = -21x + 21x + x2 + -1x2

Combine like terms: -21x + 21x = 0
21x + -1x2 = 0 + x2 + -1x2
21x + -1x2 = x2 + -1x2

Combine like terms: x2 + -1x2 = 0
21x + -1x2 = 0

Factor out the Greatest Common Factor (GCF), 'x'.
x(21 + -1x) = 0

Subproblem 1
Set the factor 'x' equal to zero and attempt to solve:

Simplifying
x = 0

Solving
x = 0

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

Simplifying
x = 0
Subproblem 2
Set the factor '(21 + -1x)' equal to zero and attempt to solve:

Simplifying
21 + -1x = 0

Solving
21 + -1x = 0

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

Add '-21' to each side of the equation.
21 + -21 + -1x = 0 + -21

Combine like terms: 21 + -21 = 0
0 + -1x = 0 + -21
-1x = 0 + -21

Combine like terms: 0 + -21 = -21
-1x = -21

Divide each side by '-1'.
x = 21

Simplifying
x = 21
Solution
x = {0, 21}