Let the distance between cities be S, then S/5.5-24=S/6+24.

That is, S/5.5-S/6=48.

Multiply both sides by 33 to get:

6S-5.5S=792

That is 0.5S=792.

Get S=3 168km.

Solution 2:

Let the flight speed of the plane be V when there is no wind, then (v+24)*5.5=(v-24)*6.

The solution is v=552.

Then the distance between the two cities is 6*(552-24)=3 168km.