Nonlinear problem

I was assuming that everything was scalars.

The first thing you need to do is to define s SCALAR objective function. That scalar function needs to be convex. If it is convex, it might be possible to use CVX, but not necessarily so.

If you have a scalar objective function, but it is not convex, you can consider using YALMIP. Until you have a scalar objective function, you are not ready to perform optimization.