The relevant formula that we can use in this situation is:
v = v0 + at
where v is the final velocity, v0 is the initial velocity = 20 km/hr, a is acceleration at 3.4 m/s^2 and t is time at 20 seconds
First convert v0 to m/s, v0 = 5.56 m/s
v = 5.56 m/s + 3.4 m/s^2 * 20 s
v = 73.56 m/s
Then convert back to km/hr:
v = 264.82 km/hr