a function f(x) is defined as f(x)= -8x^2. What is f(-3)?
All you need to do is substitute -3 in place of the x.
substitute -3 for every x in f(x) and simplify.
by now you've probably done it, but watch the "-" signs carefully
f(-3) = -8 * (-3)^2 = -8(9) = -72