Just use the Pythagorean Theorem. To find the measure of segment AB, just count, or you could use subtraction: point A is on 8, and point B is on 2. 8-2=6 (You should get the same thing is you count.) The other leg, segment BC, is 3 units. (On that one, you could count or use the coordinates on the x axis.) The Pythagorean Theorem says a squared (a^2) + b squared (b^2) = c squared (c^2).
a and b are the legs: 6 and 3, and c is the hypotenuse: segment AC. *Remember when you square something, you multiply it by itself.* So...
6×6=36 (segment AB)
3×3=9 (segment BC)
36+9=45
You're not done yet. 45 is c^2, not segment AC. You have to take the square root of 45, which is 6.7
Segment AC is equal to 6.7