You don't say which charge is where. Are we to assume that the first named charge, $\displaystyle 1.7\times 10^{7}$ C, is at A and the second named charge, $\displaystyle 5.3 \times 10^{7}$ C is at B?
If so this is pretty much just straight computation. The electric field at distance r from charge Q is give by $\displaystyle k_e\frac{Q}{r^2}$ where $\displaystyle k_e= 9\times 10^{9}$ $\displaystyle Nm^2/C^2$, "Coulomb's constant". Then add the field strength due to the two charges separately.
