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 ( = ).
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.
you should provide the f (t); what I chose is a convenience function. I do not believe that Mathcad is able to determine f (t) as an unknown knowing only its initial condition.