Suppose we want to find the length of the line between A and B in the diagram below.

We draw a straight line between A and B, and make this rhe hypotenuse of a right angled triangle by drawing a horizontal line from A to a point directly below B and another line down from B to a point right of A. This produces the diagram below.

We now have a right angled triangle with base 9 and height 7.

Using Pythagoras theorem gives