How can we write the following cost function in cvx?

Minimize (a+x)^u
a constant
x, u variables

I forgot to write
1<= u<= 2
Also, i simplified my cost function, if it is necessary I can write it again

x^u where x and u are both optimization variables is neither convex not concave; so can;'t be used in CVX.

Thanks for your kind reply.
Is it possible to use ADMM Algorithm? It can use for both convex and nonconvex

That really doesn’t have anything to do with CVX, so is best discussed elsewhere.