Hi, we have f(x)=2x and g(x)=x^2-15, what is (f-g)(-3). And this is a multiple choice question with the following answers :

a) -12

b) -6

c) 0

d) 30

I first found out (f-g) which = (x-5)(x+3)

Then this means that (x-5)(x+3)(-3). Since (f-g)(-3).

What next? Is this all the question is asking? I know the zeros 5 and -3 and we have the other value of -3. Now what?