Resistance of a uniform wire is given by,

$R=\frac{\rho L}{A}$.

As volume will remain constant, therefore,

$$\begin{gathered} AL=V \\ \Rightarrow A=\frac{V}{L} \end{gathered}$$

So we can write resistance as,

$$\begin{gathered} R=\frac{\rho L}{{\displaystyle \frac{V}{L}}} \\ =\left(\frac{\rho }{V}\right)L^2 \end{gathered}$$

Now for small changes we can write,

$$\begin{gathered} \frac{∆R}{R}=2\frac{∆L}{L} \\ \Rightarrow \frac{{\displaystyle ∆L}}{{\displaystyle L}}\times 100=\frac{1}{2}\left(\frac{{\displaystyle ∆R}}{{\displaystyle R}}\times 100\right) \\ =\frac{1}{2}\times 6=3\% \end{gathered}$$