The top is 9in by 4in. Its area is (9in x 4in) = 36 in² .
The bottom is the same as the top.
So the top and bottom add up to 72 in².
The front is 9in by 2in. Its area is (9in x 2in) = 18 in² .
The back is the same as the front.
So the front and back add up to 36 in² .
The left side is 4in by 2in. Its area is (4in x 2in) = 8 in² .
The right side is the same as the left side.
So the left and right sides add up to 16 in² .
All together, the six sides of the rectangular solid add up to
72 in² + 36 in² + 16in² = 124 in²