(a) Given, $F_{1} = F_{2} = F\ and\ \theta = 60{^\circ}$
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$Resultant\ force = \sqrt{3}\ F$
$\therefore Force\ on\ mass\ at\ A\ due\ to\ mass\ at\ B\ and\ C$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \sqrt{3}\left( \frac{{GM}^{2}}{L^{2}} \right)$
Centripetal force for circumscribing the triangle in a circular orbit is provided by mutual gravitational interaction.
$ie,\ \ \frac{{Mv}^{2}}{\left( L/\sqrt{3} \right)} = \sqrt{3}\left( \frac{{GM}^{2}}{L^{2}} \right)$
$or\ \ v = \sqrt{\frac{GM}{L}}$