The first things you need to do are read the CVX Users’ Guide and Why isn't CVX accepting my model? READ THIS FIRST! o learn the proper way to use CVX.
The error message you received s non-informative of the error. You need to use a PSD quadratic form rather than directly entering variable product terms. The objective function could be entered as
x(1:2)'*[1 1;1 2]*x(1:2) + x(3)^2 + 3*x(1) - 4*x(2)
although there are other ways which would also work.
The square root term needs to be be expressed as a norm. I’ll let you work out the details so that you learn something and do your own school assignments. The last term on the LHS of that constraint can be handled using quad_over_lin.
Given your homework assignment, you should not rush to make another post, as it might take you a while.