You simply need to plug the values you're given in the formula.
[tex] v = v_0+at [/tex] is the law that describes the velocity of a body subject to a constant force - and thus a constant acceleration. So, its velocity changes with a constant rate over time.
If you plug the values you're given for the initial velocity, acceleration and time, you get
[tex] v = 20+1.5\cdot 4 = 20 + 6 = 26 [/tex]