How to do it in your head:
When the second car leaves, the first car is d=rt=50×2=100 miles away.  The
second car's approach rate is 70-50 = 20 mph, and it will take him 
t=d/r=100÷20 or 5 hours.

How to do it by algebra:

Make this chart:

              distance     rate   time     
first car                  
second car               

Fill in the rates (speeds)


              distance     rate   time     
first car                   50      
second car                  70      

Let t = the time for the second car (which is what we want to find)
Then since the first car left 2 hours earlier, its time is t+2.
Fill those in


              distance     rate   time     
first car                   50     t+2  
second car                  70      t

Fill in the distances using d=rt

              distance     rate   time     
first car      50(t+2)      50     t+2  
second car       70t        70      t

When the second car catches the first car, the
distances are equal:
                 
                  50(t+2) = 70t
                50t + 100 = 70t
                     -20t = -100
                        t = 5 hours

Edwin