Jupiter's moon Ganymede orbits 1.07 × 10⁹ m from Jupiter's centre, once every 7.15 days…

Step 1. $T = 7.15 \times 86\,400\ \text{s} = 6.18 \times 10^{5}\ \text{s}$.

Step 2. $M = \dfrac{4\pi^2}{6.67 \times 10^{-11}\ \text{N m}^2/\text{kg}^2} \times \dfrac{(1.07 \times 10^{9}\ \text{m})^3}{(6.18 \times 10^{5}\ \text{s})^2} = 1.90 \times 10^{27}\ \text{kg}$.