The first variable is velocity and is represented by the letter v. It is our unknown in the question so the answer would be:
v = ?
The initial velocity is symbolized by vo and since it starts from rest it is equal to zero.
vo = 0 m/s
The acceleration is symbolized by the letter a, and is given in the problem, so the answer would be:
a = 6 m/s2
Finally, the time symbolized by the letter t is the value given in seconds in the statement.
t = 4s