Add the time they arrive at the common to individual starting points.
9.00 +[tex] \frac{x}{36} =10.00 + \frac{x}{42} [/tex]
put the like terms together;
10.00 - 9.00=[tex] \frac{x}{36} - \frac{x}{42} [/tex]
1=[tex] \frac{42x-36x}{36*42} [/tex]
1=[tex] \frac{6x}{36*42} [/tex]
6x=36*42
x=[tex] \frac{36*42}{6} [/tex]
x=252 miles