Hint 0: We have to generalised bothR.h.s and L.h.s
Hint 1 : Take C=-(a+b)
Hint 2: Now do binomial expansion for powers of 5 and 3
Hint 3: Now equate the coefficients of powers of a^i* b^j on R.h.s and L.h.s you will find that they are equal .
Hence done!