OK. I did it. I drew a triangle, then measured the length of each side.
The lengths of the sides are 3 inches, 4 inches, and 5 inches.
(7.62 cm, 10.16 cm, and 12.7 cm)
"Perimeter" means the distance all the way around it, so I added them up.
I found that the perimeter of my triangle is 12 inches. (30.48 cm)
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Now I have a question for YOU:
Why couldn't YOU do that ?
Take a pencil, paper, and ruler, draw a triangle on the paper,
then measure the lengths of the sides and add them up ?
Which part of that didn't you understand ?