Acceleration by definition is the time rate change of velocity.(velocity and speed are the same in unidirectional motion)
When you say that your acceleration is 18m/s^2 what you're saying is that there is a change of 18m/s in velocity, every second.
Since your acceleration or 'change' is positive, you know that the velocity is increasing.
The net change in velocity over 12 seconds is = 12 x 18 = 216 m/s
Since the body mentioned in the question was initially at rest, this is also the final velocity of the body.
