RT=D relates speed, time, and distance.
Their total distance traveled for each to reach the theater is 24 km.
The description does not give enough information to find easily nor know the rate of Sam or Jon. "...both travel at an average speed." Not too specific. Each moves at his own speed.
Jon j 12/60 j(12/60)
Sam s 36/60 s(36/60)
TOTAL 24
At best, you really have .
If you want to choose some quantity r for the average of their rates, then you may make
, and you could use this to find how far is Jon and is Sam from the theater.
"DIRECTLY BETWEEN THEIR HOMES"
They would each travel an equal distance d from home to theater.
Jon j 12/60 d
Sam s 36/60 d
Total 24
This way gives you , more like what you might want, for a system.
and solving the system is not necessary because enough information of where the theater is already given. kilometers. Now you could also find each of their speeds.