348
Basically there are 3 types of rectangles on a rectangular prism
Width by height
Width by length
Length by height
Find the areas of these 3 rectangles and multiply by 2. (Because there's 6 sides on a rectangular prism)
To find the areas, just plug in what you have
9(width) × 6(height) = 54
9(width) × 8(length) = 72
8(length) × 6(height) = 48
(54 + 72 + 48) = 174
174 × 2 = 348