a b c d

d

Electric field due to charge at corner A and D will cancel each other out. Electric field due to charge 2q and -q will add up. Length of diagonal is 10√2 cm. half of diagonal =5√2 cm E = $\frac{1}{4\pi\epsilon_0}\frac{3q}{(5√2 cm)^2}$ = 9 $\times 10^3$