Do not (only) post pictures, post the worksheet!. In this case it's important to know what the error message is. Here I can guess:
"Undefined variables" or so.
You are requesting the numeric answer from symbols that have not been numerically defined.
Symbolic evaluation would (most probably) have given you a (symbolic) answer.
Lesson: it is important to realize that Prime (as well as Mathcad) has a numeric engine and a symbolic engine. They can do many the same tricks, but the numeric processor requires variables to be numerically defined and understands units. The symbolic processor can happily work with undefined symbols, but cannot manipulate units (it treats them as unknown symbols).
