Actually, you don't need to find fg(x). fg(x)= f(x)g(x) and g(k)= ?
HOWEVER, if the answer in the book is "2", they mean , the composition, NOT , the product. I'm very surprised a textbook would make that error. But, again, you do not need to find , just find g(k) and then f of that value.
is easy- but you will need a logarithm.