Two wires each carry $10.0 \mathrm{~A}$ of current (in opposite directions) and are $3.0 \mathrm{~mm}$ apart. (a) Calculate the magnetic field $25 \mathrm{~cm}$ away at point $P$, in the plane of the wires. (b) What is the magnetic field at the same point if the currents instead both run to the left?