Error Symbolic

The double integral (your first error in the picture) is solved. The result is just too big to be displayed.

The solve for variables is badly formatted. You should just solve for a1 if that is what you need, so leave OFF the (a,b,x,y). You should just have

solve, a1

IF if, if a solution is found, it will be a function of a, b, x, y, q and D.

You can make that explicit by assigning the symbolic result to a function a1 of these variable, that is by writing:

a1(a,b,x,y,q,D) := <the double integral symbolic solve>

Also here be prepared that the solution may be too large to display...

Success
Luc

PS: You had symbolically derived the function w(x,y,a1,a,b). Why did you redefine it without a1,and without pi in the arguments to the (co)sine functions before taking the double integral?