This is the problem:

Find the slope of the tangent to the curve of intersection of the cylinder 4z = 5*sqrt(16 - x^2) and the plane y = 3 at the point (2, 3, 5*sqrt(3)/2).

I figured I would find d/dx of fx and plug in 2. I got it down to -5x / 2*sqrt(16 - x^2) in which then I plugged in 2 and got -5/12. I'm not sure where to go from here. Any help would be appreciated, thanks.