Manipulation for converting equality constrains from nonlinear to linear

My task involves minimizing sum(a + b) subject to the equality constraints a = v^2 and b = 3*v^3. What manipulations should I be doing so that I can represent both a & b as linear equality constraints?

minimize(v^2+3*v^3)

Equality constraints are not allowed in CVX with nonlinear expressions.

If the above formulation does not suit your needs, then perhaps your problem is not convex?

It seems like that is the case (the problem being non-convex). Maybe I need to reformulate the problem. Thanks for your help, though.