As PGStats said, look at the doc for PROC RSREG. The Getting Started example looks similar to what you need.
This (untested) code might point you in the right direction:
proc rsreg data=Have plots(unpack)=surface(3d);
model yield = temperature pressure;
Notice that the procedure automatically fits a quadratic model, so you do not need to specify the quadratic terms for temperature^2, pressure^2 or temperature*pressure.
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