The straight line distance is 5 miles from starting point.
Explanation:
Driving 3 miles north and 4 miles east and then the straight line distance will form a right angled triangle.
Where;
One leg = a = 3 miles
Other leg = b = 4 miles
Hypotenuse = c
Using Pythagoras theorem;

Taking square root on both sides

The straight line distance is 5 miles from starting point.