This is the Best answer for What is the magnitude of the magnetic field vec B at point a in the figure?
The fields are clockwise in the direction of positive current flow. With currents being equal and opposite, the total field along the x axis points along +x at all 3 points.
Field from each wire B0 = mu0*I/(2pi*d)
At point b:
d = 0.01 ==> B0 = 0.0002 T
Fields are both along x axis ==> total B = 2B0 = 0.0004 T
At points a and c:
d = sqrt2*0.01 ==> B0 = 0.0002/sqrt(2) T
Fields are 45 deg to x axis ==> total B = sqrt(2)*B0 = 0.0002 T