I have the question prove by induction that 5^2n-4^n is divisible by 7.

In proving p(n+1) implies P(n) I have got:

(5^2n-4^n)+ 21(5^2n-4^n) is divisible by 7 by inductive hypothesis and since 21 is divisible by 7

Just wondering if this argument holds or if anyone has any ideas?