BOB PLEASE HELP

Use implicit differentiation to find the slope of the tangent line to the curve y/(x-2y) = y^3 +9 (1,10/21) ive done this problem around 10 times and i still cant get it.... when i did it the last time i got -9-y^3/ 3y^2 x - 8y^3 - 19 and the slope around...