How to transform this nonhomogeneous equation into a homogeneous one by change of variables?

Question

I am trying to transform $$u_t = u_{xx} - 5$$ into a homogeneous equation. I am looking for a change of variables in order to get rid of the $-5$. I tried a few things like $s=x$ and $\tau = t - 5$ for a function $\bar{u}(s,\tau)$ but this doesn't work out. I am just looking for a hint, not the full process of applying the transformation.