Length of a rectangular prism volume is 2,830.5, width 18.5, height 9

Well, you get the volume by multiplying the height, the width, and the length. So just work on it backwards.
2,830.5 divided by (9 times 18.5).
9 times 18.5 is 166.5. So that leaves us with 2,830.5 divided by 166.5, which is 17.
So, the answer is 17.

Multiply 18.5 and 9 to get 166.5. Then divide 2,830.5 by 166.5 to get 17. Your answer is 17