Answer:
G M m / R^2 = F force of attraction for proton
K e^2 / r^2 = F force of attraction between 2 protons
G M m / R^2 = K e^2 / r^2
r^2 = K e^2 R^2 / (G M m)
r^2 = 9E9 * (1.6E-19)^2 * (6.37E6)^2 / (6.67E-11 * 5.98E24 * 1.7E-27)
r^2 = 9 * 2.56 * 40.6 / (6.67 * 5.98 * 1.7) * 10^-19
r^2 = 1.38 * 10^-30
r = 1.17E-15 m