This is an example of the Pythagorean Theorem. We take Point A and then draw a vertical line down. We then take Point B and draw a horizontal line. You can now now see the right triangle that has formed. Count the square blocks from Point A until the intersection (the line containing Point B). Now count the blocks from Point B to the intersection with the vertical line containing Point A. The Pythagorean Theorem works like this:
this basically means that you take the distance from Point A and the intersection (the horizantal line containing Point B) and square (multiply it by itself) it. In this case, that's 5 x 5 = 25. Then you the same for Point B. This is 3 x 3 = 9. Then, you add the two together for a sum of 34. Since it states, that this is c squared (listed in the equation above), you square root it for an answer of 34 square rooted (5.831 rounded to the nearest thousandth). I hope this helps!