As FM already wrote it looks that you attached a wrong, empty worksheet.
To enter an equation you should not use the normal equal sign - this is used for numeric evaluations only and the question mark means that the expression can not be evaluated numerically (because it contains a variable like the L you are talking about which is not defined).
For equations you'd like to be solved you have to use the (big fat) Boolean equal sign. Depending on the keyboard language you get it by typing "Ctrl =" or "Ctrl +" (without the quotes) or from the ribbon menu:
