We know that
• Sean runs 6 miles per hour (rate).
,
• Sean's initial condition is 0.25 miles.
,
• Darryl runs 0.7 miles per hour faster than Sean (we have to sum).
Sean's expression would be 6x + 0.25.
Darryl's expression would be 6x + 0.7x = 6.7x.
Now, we make them equal

Hence, the right answer is C.