Newton invented calculus to prove his ideas. Planets orbit the sun in curved paths. Newton deduced that a force was acting onthe planets. If no force was present, planets would travel in straight lines. The more mass an object has, the harder it is to move.
More force is needed to give the same acceleration to a more massive object.
Twice the mass requires twice the force. The sun is much more massive than the planets. Both feel the same force, but the planets have greater accelerations. R squared G is a gravitational constant. Newton calculated the gravitational acceleration at the distance of the moon and compared it to the moon's actual orbital acceleration.