Mangetic field B due to a current carrying conductor at a distance d from it is given by
= Mu I / 2π d I = current Mu = permeability in the medium (vacuum)
Suppose P is to the left of Q. Midpoint is at d = 5/2 m = 2.5 m
If we take the wire P, direction of B due to P at midpoint between wires is upwards on the plane of paper. I assume that the current is coming out of plane of paper. Its strength is
B = 4 π * 10^-7 * 2.5 / 2 * π * 2.5 = 2 * 10^-7 Tesla, upwards
If we take the wire Q, direction of B due to Q will be downwards on the plane of paper.
B = 4 π * 10^-7 * 5 / 2 * π * 2.5 = 4 * 10^-7 Tesla, downwards
Resultant Magnetic field is : 4 * 10^-7 downwards