Is that correct forwhat? It is not the tangent plane but you don't seem to mean that since you ask about the tangent plane below. What did you intend the plane z= y to represent.

(It is not the tangent plane because a horizontal plane is of the form z= constant.)

problem: the equation of a tangent plane requires me to compute f(a,b) but I can't do this since Q is an implicit function.

I haven't learned about vectors yet