Let $f: \mathbb{R} \rightarrow \mathbb{R}$ be a continuously differentiable function such that $0<c_1 \leqslant f^{\prime}(x) \leqslant$ $c_2$ for all $x$. Show that, if $F$ is an $s$-set in $\mathbb{R}$, then $\underline{D}^s(f(F), f(x))=\underline{D}^s(F, x)$ for all $x$ in $\mathbb{R}$, with a similar result for upper densities.