As volume is constant, so, length doubled and area reduces to half.

Resistance of a wire is given by $R=\frac{\rho l}{A}$, where, $\rho$ is resistivity of a material.

Here, $R_1=\frac{\rho l}{A}$ and $R_2=\frac{\rho \left(2l\right)}{{\displaystyle \frac{A}{2}}}=4R$

So percentage increase in the resistance will be $=\frac{4R-R}{R}\times 100\%=300\%$