Two parallel conductors carry current in opposite directions as...

The magnetic field at $\mathrm{C}$ due to first conductor is \mathrm{B}_{1}=\frac{\mu_{0}}{2 \pi} \frac{\mathrm{I}}{3 \mathrm{ d} / 2} (since, point $\mathrm{C}$ is separated by $\mathrm{d}+\frac{\mathrm{d}}{2}=\frac{3 \mathrm{ d}}{2}$ from 1 st conductor). The direction of field is perpendicular to the plane of paper and directed outwards. The magnetic field at $\mathrm{C}$ due to second conductor is $\mathrm{B}_{2}=\frac{\mu_{0}}{2 \pi} \frac{10}{\mathrm{ d} / 2}$ (since, point $\mathrm{C}$ is separated by $\frac{\mathrm{d}}{2}$ from 2 nd conductor) The direction of field is perpendicular to the plane of paper and directed inwards. Since, direction of $\mathrm{B}_{1}$ and $\mathrm{B}_{2}$ at point $\mathrm{C}$ is in opposite direction and the magnetic field at C is zero, therefore, $$\begin{array}{c}\mathrm{B}_{1}=\mathrm{B}_{2}\\ \frac{\mu_{0}}{2 \pi} \frac{\mathrm{I}}{3 \mathrm{ d} / 2}=\frac{\mu_{0}}{2 \pi} \frac{10}{\mathrm{ d} / 2} \end{array}$$ On solving $\mathrm{I}=30.0 \mathrm{ A}$