Find the exact value of each expression.
Cos 3pi/8

 cubic feet

Step-by-step explanation:

If these are the width, length and height of a rectangular box (such details were not specified in the question), the volume is calculated by multiplying the three quantities because the volume of a rectangular prism is : width x height x length.

The easiest way to do it when mixed numbers are involved (specially when the denominators of the fractions are "4" like in your case), is to convert them to decimal form and then do the multiplication:

3 1/4 = 3.25

3 3/4 = 3.75

now do the product of the three quantities:

3 x 3.25 x 3.75 = 36.5625 = 36 + 0.5625

now convert 0.5625  into fraction form: 9/16 and you get the final answer (remember to include the cubic form of the units "ft" since each quantity you multiply carries a feet unit:

36  9/16  cubic feet