f(0)=-0.2(x+5)(x-5)

Solution for f(0)=-0.2(x+5)(x-5) equation:

Simplifying
f(0) = -0.2(x + 5)(x + -5)

Reorder the terms for easier multiplication:
0f = -0.2(x + 5)(x + -5)

Anything times zero is zero.
0f = -0.2(x + 5)(x + -5)

Reorder the terms:
0 = -0.2(5 + x)(x + -5)

Reorder the terms:
0 = -0.2(5 + x)(-5 + x)

Multiply (5 + x) * (-5 + x)
0 = -0.2(5(-5 + x) + x(-5 + x))
0 = -0.2((-5 * 5 + x * 5) + x(-5 + x))
0 = -0.2((-25 + 5x) + x(-5 + x))
0 = -0.2(-25 + 5x + (-5 * x + x * x))
0 = -0.2(-25 + 5x + (-5x + x2))

Combine like terms: 5x + -5x = 0
0 = -0.2(-25 + 0 + x2)
0 = -0.2(-25 + x2)
0 = (-25 * -0.2 + x2 * -0.2)
0 = (5 + -0.2x2)

Solving
0 = 5 + -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 = 5 + -0.2x2 + 0.2x2
Remove the zero:
0.2x2 = 5 + -0.2x2 + 0.2x2

Combine like terms: -0.2x2 + 0.2x2 = 0.0
0.2x2 = 5 + 0.0
0.2x2 = 5

Divide each side by '0.2'.
x2 = 25

Simplifying
x2 = 25

Take the square root of each side:
x = {-5, 5}