Answer
$V = \frac{1}{4\pi~\epsilon_0}~\frac{Q}{R}$
Work Step by Step
We can write the general expression for the electric potential at point P due to a system of charges:
$V = \frac{1}{4\pi~\epsilon_0}~\sum \frac{q_i}{r_i}$
Note that electric potential is not a vector, so electric potential does not depend on the direction from point P to a charge.
In this case, $\sum q_i = Q$ while the distance between all the charge and point P is $R$
Therefore:
$V = \frac{1}{4\pi~\epsilon_0}~\frac{Q}{R}$
