The point a has an x-ordinate of 3. Go straight to the left until your are under point C. You are now at an x-ordinate of -5. The horizontal distance from A to C is then 5- -3 = 8. Likewise, the vertical distance from A to C (the y-ordinate distance) is 5 - -1, or 6. Now we have a right triangle with legs 6 and 8. The Pythagorean Theorem then tell us the hypotenuse (straight line distance from A to C) is the square root of 6^2 + 8^2, or the square root of 100, which is 10..