(3f-9)(3f+9)=

Solution for (3f-9)(3f+9)= equation:

Simplifying
(3f + -9)(3f + 9) = 0

Reorder the terms:
(-9 + 3f)(3f + 9) = 0

Reorder the terms:
(-9 + 3f)(9 + 3f) = 0

Multiply (-9 + 3f) * (9 + 3f)
(-9(9 + 3f) + 3f * (9 + 3f)) = 0
((9 * -9 + 3f * -9) + 3f * (9 + 3f)) = 0
((-81 + -27f) + 3f * (9 + 3f)) = 0
(-81 + -27f + (9 * 3f + 3f * 3f)) = 0
(-81 + -27f + (27f + 9f2)) = 0

Combine like terms: -27f + 27f = 0
(-81 + 0 + 9f2) = 0
(-81 + 9f2) = 0

Solving
-81 + 9f2 = 0

Solving for variable 'f'.

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

Add '81' to each side of the equation.
-81 + 81 + 9f2 = 0 + 81

Combine like terms: -81 + 81 = 0
0 + 9f2 = 0 + 81
9f2 = 0 + 81

Combine like terms: 0 + 81 = 81
9f2 = 81

Divide each side by '9'.
f2 = 9

Simplifying
f2 = 9

Take the square root of each side:
f = {-3, 3}