Dear,

when a particle is projected towards a nucleus, the condition for closest distance of approach is-

K.E of particle = P.E of system of particle and nucleus under closest distance

Therefore,

K.E of particle = 1/4πε _o * qQ/d

5 MeV = 1/4πε _o * (e)*(79e)/d

5x10 ⁶ x1.6x10 ^-19 = 9x10 ⁹ x [(1.6x10 ^-19 x79x1.6x10 ^-19 )/ d]

d = 23* 10 ^-15

d = 23 dermi