Two identical long wires, 1.5 cm apart, push each other apart with a force of 6.0 mN on each metre…

Step 1. The current. $I^2 = \dfrac{(F/L)\,d}{2 \times 10^{-7}\ \text{T m/A}} = \dfrac{6.0 \times 10^{-3}\ \text{N/m} \times 0.015\ \text{m}}{2 \times 10^{-7}\ \text{T m/A}} = 450\ \text{A}^2$. So $I = 21.2$ A.

Step 2. The direction. The wires repel, so the currents are in opposite directions.

Step 3. The force on $3.5$ m. $F = 6.0\ \text{mN/m} \times 3.5\ \text{m} = 21.0$ mN.