I have to find the area of the largest possible rectangle that can be inscribed under the curve y=e^(-x^2) in the first and second quadrants.

How do I do this?

It is symmetrical, so one could ignore the second quadrant, and work on the largest rectangle in the first quadrant.

Area= xy= x e^(-x^2)

dArea/dx=0 = ....you do it, solve for x.