just add up the lengths of the three sides
length of AB =

length of BC

length of CA

(yeah, it's with pythagoras)
let's sum everything up
length AB+BC+CA = 21.011
the units would just be considered as "length units" I guess
hope it helps. have a nice day.
feel free to ask questions.
and maybe, if I'm feeling really nice and a lil bored, i leave greeting answers under yours to enable posters to give gives brainliests. if theres no 2and answer at least