We can form a right triangle with the segments AC (one of the legs), BC (the other leg), and AB (the hypotenuse).
The Pythagorean theorem states:
where a and b are the legs and c is the hypotenuse of a right triangle.
From the graph, the length of AC is 8 units, and the length of BC is 2 units. Substituting in the Pythagorean theorem we get: