Nevermind I've worked it out,
If anyone is interested:
Suppose mn/a. Then there exists an interger q such that mn=qa. Using linear combinations we get hcf(a,m)=ra+sm for some intergers r and s, and hcf(a,n)=ta+bn for some intergers t and b.
Hcf(a,m) x hcf(a,n) = (ra+sm)(ta+bn)
=rt(a^2)+rabn+smta+sqab using (mn=qa) from above
So that (hcf(a,m)xhcf(a,n))/a