A wire has a resistance of 7 Ω. It is stretched evenly until it is 1.5 times its first length…

$R = \rho\,\dfrac{L}{A}$: the resistance grows with the length, falls with the area, and depends on the material through its resistivity $\rho$ (in $\Omega\,\text{m}$). Change square millimetres to square metres: $1\ \text{mm}^2 = 10^{-6}\ \text{m}^2$.