$\text{Potential due to charge on Sphere }V_1= \frac{q}{4\pi\epsilon_0R}$

$\text{Potential due to charge Q placed at Sphere } V_2= \frac{Q}{4\pi\epsilon_0R/2} = \frac{2Q}{4\pi\epsilon_0R} $

$\text{Net potential } V = V_1 + V_2 = \frac{2Q}{4\pi\epsilon_0R} + \frac{q}{4\pi\epsilon_0R}$