The formula:
A = bh + L (s1 + s2 + s3)
A: area
b: base
h: height
L: length
s1: side 1 (cross-sectional area)
s2: side 2 (cross-sectional area)
s3: side 3 (cross-sectional area)
Here’s an example (see attached image)
A = (4 x 6) + (12 x [7 + 7 + 4])
A = (24) + (12 x 18)
A = 24 + 216
A = 240cm^2
I hope this helped? Comment if you need more explanation or anything!
