The mutual forces of gravity between the Earth and an object on or near
its surface are (mass of the object) x (acceleration of gravity on Earth).
These two forces are equal, and we call their strength the "weight" of
the object. It's the number shown on the scale as long as nobody has
their thumb on the scale. In this problem, the force is 45N . (That's
about 10.12 pounds.)
The acceleration of gravity on Earth is about 9.8 meters per second² .
So 45N = (mass in kilograms) x (9.8 meters per second²)
Divide each side by 9.8 : Mass = 45/9.8 = 4.59 kilograms (rounded)