A car initially at rest accelerates in a straight line at 18 m/2^2. what will be its speed after 12 seconds?

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.