SAS is not really a numerical programming language, like MathCAD. You could get an approximation to the solution using a search algorithm like, for instance, Newton-Raphson, to compute local maxima across the range of x and then using successively smaller intervals to circle in to get an answer to a certain level of precision.
I don't recognize the functional form to visualize the response surface. If there is not a unique solution, the search algorithms may not converge to the answer you were looking for.