A student plots the voltage across a constantan wire against the current through it…

Step 1. The resistance. $R = \dfrac{3.0\ \text{V} - 1.2\ \text{V}}{0.50\ \text{A} - 0.20\ \text{A}}$ $= \dfrac{1.8\ \text{V}}{0.30\ \text{A}} = 6.0\ \Omega$.

Step 2. The resistivity. $\rho = \dfrac{R\pi d^2}{4l}$ $= \dfrac{6.0\ \Omega \times \pi}{4 \times 0.750\ \text{m}}$ $\times (0.28 \times 10^{-3}\ \text{m})^2$ $= 4.93 \times 10^{-7}\ \Omega\,\text{m}$.

Step 3. The power. $P = VI = 3.0\ \text{V} \times 0.50\ \text{A}$ $= 1.5$ W.