Newton's claim was outrageous and correct: the apple and the Moon obey the same rule. Every pair of masses in the universe attracts, and the size of that pull is $F = \dfrac{G m_1 m_2}{r^2}$ — read aloud, F equals big G, m-one m-two, over r squared.

Letter by letter, because a formula with undefined letters is just decoration. $F$ is the gravitational force in newtons, and it is the SAME size on both bodies — the Earth pulls you down exactly as hard as you pull the Earth up. $m_1$ and $m_2$ are the two masses in kilograms; the subscripts 1 and 2 only name the two bodies, and swapping them changes nothing. $r$ is the distance between their centres, in metres — centre to centre, never surface to surface. And $G = 6.67 \times 10^{-11}\ \mathrm{N \cdot m^2/kg^2}$ is the universal constant: the same number here, in this galaxy, and in every other. Whichever letter the question leaves blank is the one you solve for.