Calculate the voltage at a point

[Colts]

Homework Statement

Calculate the electric potential at (a) point P in the first figure, and (b) point M in the second.
http://www.usi.edu/science/physics/pickett/206/5p3f1.jpg

Homework Equations

V=\frac{q}{4\piεr}
E=\frac{q}{4\piεr^{2}}

The Attempt at a Solution

I'm not sure if E is really necassaryt, but I wasn't sure how else to get the direction right. So I did the vector stuff with E and got
\vec{E}=<0,\frac{-q}{2\pi\epsilon(.75)a^{2}}>

Now I'm stuck and don't know how to get to voltage

 

[ap123]

You don't need the electric field - just use the definition of V.

 

[Colts]

Is it as easy as adding the two V's up?
Like
V_{1}+V_{2}=V
So the answer would be
V=\frac{q}{2\pi\epsilon a}

Does direction not matter?

 

[ap123]

Yes, you just add them up.
The potential is a scalar, not a vector, so it doesn't have a direction.