Since the voltage is turned on at t = 4, we need to use u(t − 4). We multiply by 12 since that is the voltage.

We write the function as follows:

`V(t)=12·u(t-4)`.

Here's the graph:

12345678-124681012tV(t)Open image in a new page

Graph of `V(t)=12·u(t-4)`, a shifted step function.