add all the sides up and im pretty sure if u only have 2 sides filled out then add those and subtract it by 180 and that's the last side
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By adding the three sides.
Add up the sides.
-- Measure or calculate the length of each of its 3 sides. -- Add the lengths of its 3 sides. -- The sum is the perimeter of the triangle.
The perimeter of a triangle is the sum of its 3 sides