Answer :
$r_1= -a \hat i+ c \hat k$
$\therefore |r_1| =\sqrt{a^2+c^2}$
$r_2= -b \hat j+ c \hat k$
$\therefore |r_2| =\sqrt{b^2+c^2}$
Net electric field at the point $(0,0,c)$
E=electric field due to change $q ,+ $ electric field due to charge $q_2$
$E =\large\frac{1}{4 \pi \in_0} \bigg[ \large\frac{q_1 (-a \hat i + c \hat k)}{(a^2 +c^2)^{3/2} }-\frac{q_2 (-b\hat j +c \hat k)}{(b^2+c^2)^{3/2}}\bigg]$