Answer:
base = 3 mm and congruent sides = 9 mm
Explanation:
let the length of the third side be x then the length of the congruent sides are 3x
the perimeter is the sum of the 3 sides
given perimeter = 21 , then
3x + 3x + x = 21
7x = 21 ( divide both sides by 7 )
x = 3
then third side is 3 mm and congruent sides = 3 × 3 = 9 mm