You can solve this by using The Universal Gravitation Equation:
F = Gm1m2/r^2
where G is gravitational constant 6.67x10^-11, m1 and m2 are the masses and r is the distance between the two masses.
It is also given that Y=3Z.
Also, don't forget to convert 2100 km into meters = 2.1 x10^6 meters
Substituting the values we have:
6.2x10^8 = (6.67x10^-11)(3Z)(Z)/(2.1x10^6)²
Z = 3.697x10^15 kg
Y = 3Z = 1.11 x10^16 kg
The answer is B.
