Along the x-axis, three charges $\large\frac{q}{2},$$-q$ and $\large\frac{q}{2}$ are placed at $x=0,x=a$ and $x=2a$ respectively. The resultant electric potential at a point P located at a distance r from the charge -q( a << r) is ($ \varepsilon _0 $ is the permittivity of free space)