To find the surface area of a prism, we must find the area of each side and add them up.
Let's first find the area of the triangular base.
The area of a triangle is

Where b is the base length and h is the height.
The base length of the triangle is 10.5. The height is 10.

The triangular base has an area of 52.5 cm².
The triangular top must also have the same area. Thus, it also has an area of 52.5 cm².
Now we to find the length of the 3 rectangles on the side of the prism.
The formula for the area of a rectangle is

where

is the width and

is the length.
All rectangles have a width of 8. They all have different lengths. If we find the area of all the rectangles, we have the following:



Now, add up ALL the areas we found. We'll have our surface area once we do this.

The surface area is 385 cm². Hope this helps! :)