Two ways to work it:

First way:

At 9am Susan has been driving 2 hours since 7AM at the rate of 45 mph so she is 
90 miles from Marion when Marion starts.  Marion's catch-up rate is 60-45=15
mph. Since TIME = DISTANCE/RATE, it will take Marion 90/15 = 6 hours to catch up
and pass Susan.

Second way:

Make a chart.  Fill in the rates, and x for Marion's time.  Susan's time
traveled when Marion catches up is 2 hours longer or x+2

          Distance = Rate · Time 
Susan                 45     x+2  
Marion                60      x

Now fill in the distances by multiplying the rates times the times.

          Distance = Rate · Time 
Susan     45(x+2)     45     x+2  
Marion       60x      60      x      

Their distances are the same, so set them equal:

     45(x+2) = 60x
      45x+90 = 60x
          90 = 15x
           6 = x

Edwin