Question

If xy=4 and dy/dy=3 then find dx/dt when x=4?

Answer 1

Your statement dy/dy = 3 must be wrong.
Did you mean dy/dt = 1?
I will assume you did.

dx/dt = dx/dy * dy/dt
= (dy/dt) / (dy/dx)
dy/dx = -4/x^2 = -1/4 when x=4

Therefore dx/dt = 3/(-1/4) = -12 at x=4

Answer 2

Oh my bad. I meant dy/dt.

I understand now. Thanks!