In the figure below, all the currents shown are infinitely long and directed to the left. The three currents are not equal. The wire in the middle is bent into a semicircle of radius $R.$ The distance between the upper wires is a and the distance between the lower wires is $b.$ What is the magnitude and direction of the magnetic field at the point indicated by the black dot $($the center of the semicircular arc of $I_2)?$