A copper wire is 25 m long, with an area of cross-section of 0.75 mm²…

Step 1. The area. $0.75\ \text{mm}^2 = 0.75 \times 10^{-6}\ \text{m}^2$.

Step 2. The resistance. $R = \dfrac{1.68 \times 10^{-8}\ \Omega\,\text{m} \times 25\ \text{m}}{0.75 \times 10^{-6}\ \text{m}^2} = 0.56\ \Omega$.