Gravitational force acting on mass m due to mass M can be calculated by formula:
F = GMm/r²
where,
F is the gravitational force
G is the gravitational constant which is equal to 6.67x10∧-11
M and m are the masses of given objects
r is the distance between point masses M and m.
From the given condition, when the mass m is placed such that net gravitational force acting on it due to masses M and 3M is zero, we have:
r= L/3
therefore,
F= 9GMm/L²
The magnitude of force acting due to mass M on mass m will be 9GMm/L².