The electric field intensity at a point p is given by E = kQ1/d12 + kQ2/d22 where k is the coulomb constant, Q1 and Q2 the charges in coulombs, d the distance from each Q to the point P.
If E = 0, kQ1/d12 = kQ2/d22. As the k's cancel, and d1 = 0.255 m, after substituting the values for Q1 and Q2 and solving for d22 gives
d22 = -5.22 x 10-9C x 0.2552m2 / -3.33 x 10-9C = 0.1019 m2
Taking the square root of d22 = 0.319 m