Explanation:
There are TWO triangles with base = 10 height = 24
area of a triangle = 1/2 b* h = 1/2 (10)(24) = 120 cm^2
TWO of them totals 240 cm^2
then there is also THREE sides
10 x 20 + 26 x 20 + 24 x 20 = 1200 cm^2
Add the two triangles to this to get the total surface area = 1440 cm^2