a name is required here

You should NOT use global assignments ( = ), but boolean equality to define the equations in your solve block. Type [CTRL =] to get those. They show as a bold equals sign ( = ).

Success!
Luc

Additional remarks:

1) You have to only use the function names without the argument parenthesis (use "C" and not "C(t)")

2) You have only 5 equations but 6 functions to solve for.