Question

HELP PLEASE!!!!! CALCULUS
the base of a solid is the region enclosed by the graph y=e^-x, the coordinate axes and the line x=3. if all the plane cross sections perpendicular to the x-axis are squares, then its volume is
a) (1-e^-6)/2 b)(1/2)e^-6 c)e^-6 d)e^-3 e)1-e^-3