in short, the volume of a "rectangular prism" is flatly just the product of its length and widht and height, now, we have all three here, so let's just get the product.
firstly we'll convert the mixed fraction to "improper", and then get the product,
[tex]\bf \stackrel{mixed}{2\frac{1}{4}}\implies \cfrac{2\cdot 4+1}{4}\implies \stackrel{improper}{\cfrac{9}{4}}
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4\cdot 2\cdot \cfrac{9}{4}\implies \cfrac{4}{1}\cdot \cfrac{2}{1}\cdot \cfrac{9}{4}\implies \cfrac{72}{4}\implies \stackrel{ft^3}{18}[/tex]
