What is the length of side BC of triangle ABC if AB = AC = 28 mm and the perimeter is 72 mm?

The perimeter of a triangle is the sum of all its sides. Here, the perimeter is 72 mm, and two sides are equal (AB = AC = 28 mm). Let BC = x. Then 28 + 28 + x = 72. Solving for x: x = 72 - 28 - 28 = 16. Thus, BC = 16 mm.