Question

F(1)=16 f(n)=(n-1)+2n what is f(4)

Answer 1

F(1) = 16
f(n) = (n - 1) + 2n
f(4) = 64
f(4) = (4 - 1) + 2(4)
f(4) = 3 + 8
f(4) = 11

Answer 2

F(4)=(4-1)+2*4=11...

This seems a bit too easy though, did you mean f(n)=f(n-1)+2n ? In this case :
f(1)=16
f(2)=16+2*2=20
f(3)=20+2*3=26
f(4)=26+2*4=34