-5(x-2y)^20 -4(5x-13y)^39 +3(5x-13y)^39 -20(x-2y)^20
=-25(x-2y)^20 -(5x-13y)^39
=-25(5-4)^20-(25-26)^39
=-25*1^20-(-1)^39
=-25*1-(-1)
=-25+1
=-24