Explicație pas cu pas:
4^n=(2^2)^n=2^(2n)
81=3^4
2^(2n)*(81+56-56)<=12^4
2^(2n)*3^4= (2^2*3)^4
2^(2n)*3^4=2^8*3^4
2n= 8, n=8:2, n= 4
