At time t, Jack’s position is 5t miles east of X while Jill’s position is 8+4t miles south of X.
The distance s between them is √((5t)²+(8+4t)²)=√(41t²+64t+64), by Pythagoras.
ds/dt=(1/2)(82t+64)/√(41t²+64t+64) is the rate of change (speed) of distance between them.
After an hour, t=1, this expression has a value (1/2)(146)/√169=(1/2)(146)/13=73/13=5.62mph.