E = (kq)/r^2
Where k is the Coulomb constant 9.0 x 10^9 Nm^2/C^2, q is the charge at some point away, and r is the distance between the charge and the observation point.
Now, E is a vector, so what you need to do is determine the magnitude and direction of E due to each charge, then add the E values vectorally. The rule for assigning the direction to E is simple: The E field lines always radiate outward from a positive charge, and radiate inward to a negative charge. (So E due to the charge at point A is downward, and E due to the charge at B is to the left.)
I'll leave it here for now. If you have problems adding the vectors (most students have forgotten most of what they know about adding vectors by the time they start electric fields) just let me know.