Assume that a planet just like Earth orbits the bright star named Sirius. If this Earth-like planet orbits with a semimajor axis of 1 AU and an orbital period of 7 months, what is the mass of Sirius?

2.94kg

Explanation: Using Kepler's law of planetary motion which states that, the square of the orbital period of a planet is proportional to the cube of the semi major axis of it's orbit.

Sirius is the name given to the brightest star in the sky

It could be literally interpreted as;

1/mass of planet = (orbital period in years^2 ÷ semi major axis in AU^3)

Orbital period = 7months is equivalent to 7/12 = 0.5833 years

Semi major axis = 1AU

Therefore, mass of Sirius is given by:

1/Mass = (orbital period^2 ÷ semi major axis^3)

1/Mass = (0.5833^2 ÷ 1^3)

1/Mass = 0.3402 ÷ 1

1/Mass = 0.3402

Therefore,

Mass = 1/0.3402 = 2.94kg

a and b sometimes have a positive value