If I understand correctly, you need to first solve for "y" in an equation.
You can type the following into the natural language input
"Solve for y in x^2-10y^2=(X^3-12y^2-x^3)^2"
and it will show you how to use the Solve function:
Solve[x^2 - 10*y^2 == (-x^3 + X^3 - 12*y^2)^2, y]
