to find the distance of 2 points, you can use the distance formula:
d = sqrt([x2 - x1]^2 + [y2 - y1]^2)
when substituting the values, think of it as one coordinate being x1 and y1 and the other as x2 and y2.
for example, A(3, 6) is x2 and y2 while B(4, -2) is x1 and y1. (this is just how i did it, you can make A as x1 and y1 while B can be x2 and y2, whatever you prefer!)
next, substitute the values. if A(3, 6) is x2 and y2 while B(4, -2) is x1 and y1, it would be this:
d = sqrt([3 - 4]^2 + [6 - (-2)]^2)
from then on, evaluate. the distance from A to B is sqrt(65) units long.
do the same thing with the distance from B to C and C to A using the formula. i attached my work in the image below! if you need more help, feel free to message me :)
(sorry if this was confusing lol)