What is 5x^2+4x=80 times x^2-5x-24=0

(5x²+4x-80)(x²-5x-24)=5x²(x²-5x-24)+4x(x²-5x-24)-80(x²-5x-24)
{distributive property of multiplication}
                                  =5x^4-25x³-120x²+4x³-20x²-96x-80x²+400x+1920
                                  =5x^4-21x³-220x²+304x+1920
This is the answer.