[Physics] Finding and plotting the current graph from a voltage graph (piecewise function)

Question

Problem description:

My attempt at making the piecewise function:

Then, in order to find i(t), I took the derivative of 80x/3 and -80x/3, with respect to x, and multiplied C. Obviously, I'm left with a unknown variable C, so I'm not sure how to proceed from here. Am I supposed to find C first and then take the derivative? (how do I do that? I've been searching the web for this possibility and can't seem to find anything)

Answer 1

C stands for the capacitance of the capacitor. According to the circuit diagram, it is given by 5 F.