Kelly drove 9 +12 miles =21 miles
This took this amount of time:
21 miles/ 42miles/hr = 1/2 hr = 30 minutes
Brenda drove sqrt(9^2 + 12^2) = 15 miles
This took this amount of time:
15 miles / 45 miles/hr = 1/3 hr = 20 minutes
So Brenda arrived 30 -20 = 10 minutes before Kelly