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
