Answer:
Newton discovered the universal law of gravitation by using his basic concept of the gravity, when he saw an apple falling from the tree and then he start thinking about the nature of the forces.
The newton's law of the motion basically defines that when an object is at the state of rest , then it always be in the rest position and if the object is in the state of moving then, it will always be in that particular position. Therefore, by using this concepts the newton start developed the universal law of gravitation.