First multiply 9 by 60 to find feet/minute: 9x60=540. Multiply that answer by 60 to get feet/hour: 540x60=32400. Divide that by 5,280 to get miles/hour: 32400/5280=6.14 roughly :)
We have

and

so the equation is indeed exact. So we want to find a function
such that


Integrating both sides of the first equation wrt
gives

Differentiating both sides wrt
gives

So we have

or

I believe the answer is B