You would find the slope then use the Pythagorean theorem to solve for other side. When finding the slope, in this case the negatives don't matter because they will get squared and become positive anyways.
A to B: 5/7
B to C: 2/9
A to C: 2/3
A to B: 5/7 = square root 74 roughly 8.6
B to C: 2/9 = square root 86 roughly 9.2
A to C: 2/3 = square root 13 roughly 3.6
8.6+9.2+3.6=21.4