derivatives – How can i find the equation to the tangent of a line using known points?

I have carried out the implicit differentiation of the original formula (x-y^3=2xy) to get the equation;

dy/dx = – (2y-1)/(3y^2-2x)

Now i need to find the equation of the tangent at point (-1, 1), i’ve plugged the values into the formula to get;

dy/dx = – 1/5

but have a suspicion i may be missing something.

Many thanks!