Area question

Find the area of the fiqure:
( I can't draw the fiqure BUT:
take a small envelope and open up with the flap laying flat down.
The base of the envelope is 6 ft,
the side of the envelope is 4.2
and the top where you lick it draw that part in half and that is the 3 ft.)



A = LW - 1/2bh
=(4.2ft x 6 ft) - 1/2 x 6 ft x 3 ft
= 25.2 ft - 9 ft
= 24. 3 ft
is this correct?