To find the perimeter of a triangle, you simply need to add the lengths of all three sides together.
Given that the side lengths of the triangle are 7 cm, 9 cm, and 12 cm, you can calculate the perimeter as follows:
Perimeter = 7 cm + 9 cm + 12 cm
Perimeter = 28 cm
So, the perimeter of the triangle with side lengths 7 cm, 9 cm, and 12 cm is 28 cm.