I said that [itex]A\ell=A_{\textrm{new}}\ell\left(1+\frac{x}{100}\right)\implies A_{\textrm{new}}=\frac{A}{1+\frac{x}{100}}[/itex]. Also, we know that [itex]R\propto\frac{\ell}{A}.[/itex]. Therefore,

Expand the quadradic term, you can claim that the [itex] {(\frac x {100})}^2 [/itex] term is very small and ignore it. Can you see where to go from there?