In case of slotted aloha efficiency=G*e^-G

1-chances of success on first attempt in=e^-G(according to poisson distribution)

G attempts made in time slot,P=e^-2=.135 (G=2)

2 - Probability of K collisions=(1-e^-G)^k*(.135)=(.865)^k*(.135)

3-Expected number of transmissions=e^G=e^2=7.38