Actually I'm trying to do the next step in which I am asked to substitute in rho=r/a using the dchange command in PDETools but I am having some difficulty. I am assuming I am having this problem because both the lhs and the rhs have a new variable (rho and a) but if I rewrite the substitution as r=rho*a then the outcome is not what I want either because things like u(r) become u(rho*a) whereas I want it to become u(rho). Any advice for this part as well?