"Distance" Word Problems:
http://www.purplemath.com/modules/distance.htm

first lap:
rate: 144
time: 3/60 = 1/20
distance: (1/20)(144) = 7.2 km

second lap:
distance: 7.2
rate: r
time: 7.2/r

average rate: (2*7.2)/(1/20 + 7.2/r) = 300

Solve for r.

:7