In words, the voltage has value 1 up until time t = a. Then it is turned off.

We have a "rectangular pulse" situation and need to use this formula:

V(t) = u(t − a) − u(t − b)

In our example, the pulse starts at t = 0 [so we use u(t)] and finishes at t = a [so we use u(t − a)].

So the required function is: