A circular loop has radius $R$ and carries current $I_2$ in a clockwise direction $($Fig$.$ P$28\mathrm{.}68).$ The center of the loop is a distance $D$ above a long, straight wire. What are the magnitude and direction of the current $I_1$ in the wire if the magnetic field at the center of the loop is zero?

Figure P$28\mathrm{.}68$

$I_1$