We have the following clock:
We want to find the lateral surface area. This means all the area of the lateral faces (the rectangular faces).
In order to do so we find the perimeter of the triangle and then multiply it by 12 cm:
lateral area = perimeter x 12 cm
We have that the perimeter of the triangle is
8cm + 8 cm + 8 cm = 24 cm
Then the lateral area is
lateral area = perimeter x 12 cm
↓
lateral area = 24 cm x 12 cm
= 288 cm²
Then, the lateral surface area is 288 cm²