I/m assuming that we have a trapezoid

The height = sqrt (9^2 - 7^2 ) = sqrt ( 32) = 4sqrt (2)

So....the surface area is

height * ( sum of the bases) / 2 =

4 sqrt (2) * ( 5 + 7) / 2 =

4 sqrt (2) * 12/2 =

4 sqrt (2) * 6 =

24 sqrt (2) dm^2 = about 33.94 dm^2