What simple science equation explains the relationship between mass, weight and gravity.

The force of gravity is the force with which massively large objects such as the earth attracts another object towards itself. All objects of the earth exert a gravity that is directed towards the center of the earth. Therefore, the force of gravity of the earth is equal to the mass of the object times acceleration due to gravity and also equal to the weight of the object.

F = ma

since F = W

W = ma