a) Find a parametrization of the hyperboloid $\displaystyle x^2 + y^2 - z^2 = 25$

b) Find an expression for a unit normal to this surface.

c) Find an equation for the plane tangent to the surface at $\displaystyle (x_0,y_0,0)$, where $\displaystyle x_0^2 + y_0^2 = 25$

For the parametrization I got $\displaystyle \Phi(r,\theta) = (rcos\theta, rsin\theta, \sqrt{r^2 - 25})$

is that the correct parametrization?