To calculate any speed, the equation is speed=distance/time
Basically, you first need to take your 9 hours and 15 minutes and combine them into a number with a decimal value.
So, 15*4=60
Sound familiar? Good, because 25*4=100, so they are the same here.
So, we can say 9 hours and 15 minutes is equal to 9.25
Take this now: Speed=538/9.25
You get 58.1621, which you can round to 58.2
This is, of course, your final answer, so Joe drove at 58.16 mph