# partial derivative with a restraint

Consider a radial function z=g(r). Let P be the point with polar coordinates $(r,\theta)=(2,\frac{\pi}{3})$ and suppose g'(2)=3
Evaluate: $\frac{\partial z}{\partial x}$ at P if y is held constant.
I know that give a function z=f(x,y) z can be determined given f and any two of x, y, r, $\theta$