= 7 * (x+y) n- 1 (x-y) n+ 1 (x+y) n-3 (x-y) n- 1
= 7 * (x+y) (n- 1-n+3) times (x-y) (n+ 1-n+ 1.
= 7 * (x+y) 2 times (x-y) 2
=7*x^2-y^2^2