physics help

two protons are 300fm apart how far apart would the protons need to be for the electrostatic force between them to equal the gravitational force between them when they are 300 fm apart.
i know to equate the two equations but would that not cancel the rs out this is a simple question but i cant wrap my head around it the answer on mark scheme is r=0.11m

you wouldn't be cancelling the r's out because they refer to different things.
you have G * m_p * m_p /(300 fm) ^2 = e * e/(4 * pi * epsilon_0 * r^2). The key is that the denominator in the LHS doesn't have r because that isn't a variable - we are looking for the value of r such that the electrostatic force is equal to the grav force at a set distance (300 fm).
hope this helps!

Ah i see why so then it would be 1/4pi epsilon * q = gmm/r^2 so then i get r= sqr root gmm 4piepsilon but im not getting r as 0.11m still?