Not sure ... are you talking about a 3-sided prism here?
IF you're talking about a 3-sided prism (n - 3), then I think you can find the area A of the base of the prism using A = (n) (s) (a) / 2 = 3 (s) (a) / 2 = 1.5 (s) (a), where s = 10 mm and a = 10.3827 mm.
So the area of base A = 1.5 (10) (10.3826) = some number you can compute using your calculator, where the area of the base is in units of square mm. You'll need to round your answer to the nearest thousandth to answer this part of your question correctly.
The volume V of your prism is just the area of the base A, times the height h of the prism: V = A * h = 15 * A, where your units for volume are in cubic mm (or mm cubed).
Hope this helps!