 Solution

Fix sample points $(x_i,z_i)$, $1\le i\le n$. A labeling by $h(x,z)=f(x)g(z)$ is the coordinatewise product of one labeling that $\mathcal{F}$ realizes on $x_1,\dots ,x_n$ and one that $\mathcal{G}$ realizes on $z_1,\dots ,z_n$. There are at most the product of the two numbers of choices, so