the surface area of any shape is the sum of all the area of individual faces
since the shape is a triangular prism, you should notice that there are 2 triangular faces and 3 rectangular sides.
the area of a triangle is
A=1/2 b*h
the area of a rectangle is
A=b*h
from your picture,
the base of the triangle is 9.6 in and the height is 3.94
there are 3 different rectangles with dimensions of
9.6 x 3.9
7.1 x 3.9
5.4 x 3.9
so figure out the the individual areas, add them all together,
any questions