From the picture,
x + y = 21
From definition,
speed = distance/time
The girl whose speed is 3.5 mph, walks x miles in t hours, that is:
3.5 = x/t
or
x = 3.5t
Similarly,
y = 2.5t
(they walk for the same time)
Replacing into the first equation:
3.5t + 2.5t = 21
6t = 21
t = 21/6
t = 3.5
It will take 3.5 hours until they meet up