(a+1)(a-1)(a+3)(a+5)-20 =(a+1)(a+3)*(a-1)(a+5)-20=[a^2+4a+3][a^2+4a-5]-20=(a^2+4a)^2-2(a^2+4a)-35=(a^2+4a-7)(a^2+4a+5)