Formula for orbital speed, v = √(GM/R)
Where G is the universal gravitational constant, M = Central Mass,
R = Distance between centers of Mass.
Given. v = 68 m/s, M = ? , R = 410 km = 410000 m., G = 6.674 10⁻¹¹ Nm²/kg²
68 = √(GM/R)
68 = √(6.674 10⁻¹¹ M/410000)
68² = (6.674 10⁻¹¹ M)/410000
(68² 410000) / 6.674 * 10⁻¹¹ = M
2.84 × 10¹⁹ = M
Mass of Planet Y = 2.84 × 10¹⁹ kg