find

There are some different errors you made:

Yyou have to use the boolean equal sign in the solve block if the calculation uses the variable(s) which is modiefied and searched for.

Without any iitialisation or definition T and A will be taken as the units Tesla and Ampere. If you initialize them with guess values you also have to solve for them, too

Find attached four ways to get the result:

1. Initializing and solving for all four variables. As the results are displayed in an array and they are of different dimension, we are not allowed to work with units with this approach in MC15 (in Prime it would be allowed).
   
2. turn A,T and theta into functions of y - this is the preferred method
   
3. substitute the equations by hand or with the help of Mathcads symbolics to get rid of the variables A,T,theta and merge all in one big equation. Not recommended
   
4. Using the programming approach create one function for the last expression (which should be 1)
   

So I guess 2) or 4) are the ways to go, but whatever method you use, V is not needed in the solve process and should therefore be calculated afterwards.