Physics prof: "As you learned in kindergarten,...
You solve this problem by completing the square, integrating by parts, and applying Gauss's Law using the Green's Function formalism of the Laplace Transform in the particle's k-space coordinate system. I'm disappointed that only one of you got this problem correct."
I'd be disappointed if this weren't easy for you. :-). I'm still learning. I'm very happy that it's been as relatively painless as it has thus far.
Cheers,
Scott.