How do you find the answer for f (5) when f (x) = - 3x^4+7x^2-2

You find f (5) the same way you evaluate any function. Put the number where the variable is and do the arithmetic.

When high powers are involved, it can simplify the arithmetic a little to do some factoring:

... f (x) = (-3x^2 + 7) * x^2 - 2

... f (5) = (-3*25 + 7) * 25 - 2 = - 68*25 - 2

... f (5) = - 1702