The bottom edge of the end triangle has a length that can be computed from the Pythagorean theorem.
.. √(15^2 -12^2) = √(225 -144) = √81 = 9
Then the perimeter of the end triangle is 9 +12 +15 = 36. The lateral area is the product of this and the length of the prism:
.. lateral area = 36*20 = 720
The two end triangles will have a total area of
.. end area = 9*12 = 108
The surface area is then
.. surface area = lateral area + end area
.. = 720 +108 = 828 . . . . . square units
