A copper wire 1.5 m long and 0.80 mm across is fixed, just slack, between two rigid posts at 35 °C…

Step 1. Stress. $Y\alpha\,\Delta T = 1.1 \times 10^{11}\ \text{Pa} \times 1.7 \times 10^{-5}\ \text{K}^{-1} \times 30\ \text{K} = 5.61 \times 10^{7}\ \text{Pa}$.

Step 2. Area. $\pi(0.40 \times 10^{-3}\ \text{m})^2 = 5.03 \times 10^{-7}\ \text{m}^2$.

Step 3. Tension. $5.61 \times 10^{7}\ \text{Pa} \times 5.03 \times 10^{-7}\ \text{m}^2 = 28.2\ \text{N}$.