Finding Cartesian equation of parametric surface.

[Ral]

1. The problem statement, all variables and given/known data
Find the Cartesian equation of the parametric surface: [2cos(t)cos(s), 3sin(s), sin(t)cos(s)]

Find eqn. of the tangent plane when S = 0, t = pi/2


2. Relevant equations



3. The attempt at a solution

I'm not quite sure what to do. All I've done is squared each term, which gave me.

4cos^2(t)cos^2(s)
9sin^2(s)
sin^2(t)cos^2(t)

[Donaldos]

x=2\cos t\cos s

y=3\sin s

z=\sin t \cos s

Can't you find \alpha and \beta so that x^2+\alpha y^2 + \beta z^2= constant?