Question

multiply

(x-f) (x+f) (x^2-f^2)

Answer 1

Multiply the first 2 brackets first ...
(x-f) (x+f) = x^2 - f^2 (this is the classic difference if 2 squares)
Bracket this result and multiply by the last bracket in the question ...

(x^2 - f^2)(x^2-f^2) giving ...

(x^4 - 2f^2x^2 +f^4