Answer:
5
Explanation:
it can be found in two ways
f(n)-f(n-1) is common difference of the sequence.
knowing f(n) = f(n-1) + 5
f(n)-f(n-1)=5
so 5 is difference of the sequence.
other method is
f(1)=4
f(2)=?
f(n)=f(n-1)+5 n=2
f(2)=f(2-1)+5
f(2)=f(1)+5
f(2)=4+5=9
f(2)-f(1)=9-4=5
so 5 is difference of the sequence.