We can solve the problem by using Netwon's third law:
[tex] \frac{T^2}{R^3} = \frac{4 \pi^2}{GM} [/tex]
where:
T is the orbital period of the planet
R is the average radius of the orbit (so, the average distance of the planet from the star)
[tex]G=6.67 \cdot 10^{-11} m^3 kg^{-1}s^{-2}[/tex]
M is the mass of the star.
First, we need to convert the period and the radius into SI units:
[tex]T=6.3 days = 5.44 \cdot 10^5 s[/tex]
[tex]R=10.5 \cdot 10^6 km=10.5 \cdot 10^9m[/tex]
And then, re-arranging the formula and substituting the numbers we find
[tex]M= \frac{4 \pi^2 R^3}{GT^2}=2.31 \cdot 10^{30}kg [/tex]
