To find the perimeter of the triangle, you will add the lengths of each side.
We know two of the sides are 16.2 cm. We also know that the third side is half this (16.2/2) or 8.1 cm.
16.2 cm + 16.2 cm + 8.1 cm = 40.5 cm
The perimeter of the triangle is 40.5 cm.