(x-5y)8 uses the Pascal Triangle row 1, 8, 28, 56, 70, 56, 28, 8, 1 for coefficients.
So the expansion is:
x8-8x7(5y)+28x6(5y)2-56x5(5y)3+70x4(5y)4-56x3(5y)5+28x2(5y)6-8x(5y)7+(5y)8=
x8-40x7y+700x6y2-7000x5y3+43750x4y4-175000x3y5+437500x2y6-625000xy7+390625y8.