Answer:
16
Explanation:
Let's give letters as the image. Triangle ABH is a right triangle, so you can apply pythagoream theorem. In particular, it's the classic "3-4-5" triangle (there are some triplets of integers that gives you a right triangle, and 3-4-5 is the most famous). Either way, the height of the triangle is 4.
At that point you can compute the area by multiplying base and height, and the total surface is 16.