You can use the Pythagorean theorem to solve this problem. 3 miles west would be 3 units to the left on a graph, and 4 miles north would be four units up on a graph. The last line would be the hypotenuse. Now, if you know about special right triangles, then it can be recognized that the triangle is a 3-4-5 triangle, and the hypotenuse is 5 units. Otherwise, the Pythagorean theorem can be used.
a^2+b^2=c^2
3^2+4^2=c^2
9+16=c^2
25=c^2
sqr 25=sqr c^2
5=c
