Assume that $F(x, y, z)$ is twice differentiable and that $F_z = F_{xx} + F_{yy}$. Let a be some constant and let $G(γ, s, t) = F(γ + s, γ − s, at)$. Find the value of a that makes $G_t = G_{γγ} + G_{ss}$, for any function F.
I tried solving the question by thinking that we should put $x = γ + s$. But how would $F_{γ + s}$ makes sense.
I have trouble starting the question