And the question with solution

2.  If three particles each of mass M are placed at the three corners of an equilateral triangle of side a, the forces exerted by this system on another particle of mass M placed (i) at the mid-point of a side and (ii) at the centre of the triangle are respectively: 

$$\begin{gathered} \left(a\right) 0, \frac{4 G{M}^2}{3a^2} \left(b\right) \frac{4 G{M}^2}{3a^2}, 0 \\ \left(c\right) \frac{3 G{M}^2}{a^2}, \frac{G{M}^2}{a^2} \left(d\right) 0, 0 \end{gathered}$$