the equation is if f(1)=7 and f(n)=f(n-1)-4, then find f(6)
basically we see that to get a term (f(n)), we subtraction 4 from the prevous term (f(n-1))
so see how many times we need to subtract
6th term, so 5 times (don't subtract of first tiem)
5*4=20
7-20=-13
f(6)=-13
