Question: Modifying a 3d plot

I've plotted the solution to a GLM using the plot3d function, but a portion of the plot is irrelevant to the presentation of the results. I can define the portion of the plot I want to discard based on the x and y coordinates, but I've gotten errors for every if-then statement I've tried to include. Any suggestions on how to properly integrate an if-then statement into the syntax?

Please Wait...