Let the distance between the cities be D.
Bus speed (v_b) = D / (72 minutes).
Car speed (v_c) = D / (36 minutes).
When moving towards each other, their relative speed is v_b + v_c = D/72 + D/36 = D/72 + 2D/72 = 3D/72 = D/24.
Time to meet = Distance / Relative Speed = D / (D/24) = 24 minutes.