Undefined variable MATHCAD HOMEWORK

You need to move your function definition after your definition of t₁.    Mathcad evaluates a worksheet from left to right and then top to bottom.   You have defined the function before t₁ so Mathcad hasn't yet evaluated t₁, hence the error.  

Stuart

PS, you should have asked your question as a "Discussion". By asking it as a "Document", other users can't post worksheets in response to your question.

Apart from what Stuart already said you should either assign L a value or make L a second argument of your function.

Otherwise L is taken as the volume unit liter, which sure is not what you intended.

For the future - you can create a new discussion by using: http://communities.ptc.com/community/post!input.jspa?containerType=14&container=2003