Answer
See the detailed answer below.
Work Step by Step
$$\color{blue}{\bf [a]}$$
We know that the current is given by
$$I=\dfrac{dQ}{dt}$$
where $Q=4t -t^2$, so
$$I=\dfrac{d }{dt}\left[4t -t^2\right]$$
$$\boxed{I= 4-2t}$$
$$\color{blue}{\bf [b]}$$
See the graph below,