5(p-2q)^2-20(-2p-q)^2
=5[(p-2q)-(4p+2q)^2]
=5[(p-2q)+(4p+2q)]*[(p-2q)-(4p+2q)]
=5(p+4p-2q+2q)(p-4p-2q-2q)
=5*5p*(-3p-4q)
=-25p(3p+4q)