a
Point $(0,0,-a)$ lies on $x-axis,$ therefore magnetic field due to
current along $z-axis$ is zero and due to rest two infinitely long wires is $\frac{\mu_{0}}{2 \pi} \frac{1}{a}$
Magnetic field due to current $I$ in the x-direction is along $y-direction.$
Magnetic field due to current $I$ in the ydirection is along$-ve$ $x-direction.$
$B_{\text {total}}^{\rightarrow}=\frac{\mu_{0}}{2 \pi} \frac{1}{a}(\hat{j}-\hat{i})$