As shown in the figure, two long parallel wires $($#$1$ and #$2)$ separated by a distance $d=18\mathrm{.}0cm$ carry a current $I_1=$I and $I_2=2$I respectively in opposite directions, where $I=4\mathrm{.}00A.$

$($a$)$ Determine the magnetic field at the point $P_1$ midway between the wires. $($Express your answer in vector form.$)$

$=$ T