I'm new to Mathematica and have never solved a PDE before. I tried to follow a textbook to get equation 4, but got something different with Mathematica (I got $(4\pi Dt)^{1/2}$ instead of $(4\pi Dt)^{3/2}$). May I ask why I got the wrong answer and how can I fix that? Thank you so much!
Textbook:
What I got:
