Lets label hot dogs as 'h' and soft drinks as 'd'
6h + 4d = 13
3h + 4d = 8.50
We can solve by process of elimination.
Subtract the first equation from the second one.
We get:
3h = 4.5
Solve for h
h = 1.5
That means that the hot dog costs $1.50 which is answer choice D.
Hope that helps :)