f(0)=-0.2x^2+20

Solution for f(0)=-0.2x^2+20 equation:

Simplifying
f(0) = -0.2x2 + 20

Reorder the terms for easier multiplication:
0f = -0.2x2 + 20

Anything times zero is zero.
0f = -0.2x2 + 20

Reorder the terms:
0 = 20 + -0.2x2

Solving
0 = 20 + -0.2x2

Solving for variable 'x'.

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

Add '0.2x2' to each side of the equation.
0 + 0.2x2 = 20 + -0.2x2 + 0.2x2
Remove the zero:
0.2x2 = 20 + -0.2x2 + 0.2x2

Combine like terms: -0.2x2 + 0.2x2 = 0.0
0.2x2 = 20 + 0.0
0.2x2 = 20

Divide each side by '0.2'.
x2 = 100

Simplifying
x2 = 100

Take the square root of each side:
x = {-10, 10}