Answer
$M = m$
Work Step by Step
Since the net gravitational force on the central sphere is zero, the upward force due to the sphere of mass $M$ must be equal in magnitude to the downward force due to the two spheres of mass $m$
Let the distance from the central sphere to the other spheres be $d$
We can find $M$ in terms of $m$:
$F = \frac{G~M~m_4}{d^2} = 2 \times\frac{G~m~m_4}{d^2}~sin~30^{\circ}$
$M = 2~m~sin~30^{\circ}$
$M = m$
