Find the coefficient of y^4 in the expansion of (2y-5)^7.
128 y^7 - 2240 y^6 + 16800 y^5 - 70000 y^4 + 175000 y^3 - 262500 y^2 + 218750 y - 78125
it said that 70000 is incorrect. Thank you, though.
