This is straightforward, really, but . . . .

3 charges,

A at point (0,0) (charge +Q),

B at point (-a, a) (charge -3Q) &

C at point (a,a) (charge +Q)

I have found the magnitude of the force acting on C due to the other charges (((sqrt 2)/4,∏ E0) * Q^2/a^")

but need to find the unit vector which specifies it's direction.

I need to find the displacement vectors for each of these other points B & C.