An object is traveling at a constant velocity of 15 m/s when it experiences a constant acceleration of 3.5 m/s2 for a time of 11s. What will its velocity be after that acceleration
-- So it gains 3.5 m/s of speed every second. -- In 11 sec, it'll gain (3.5 x 11) = 38.5 m/s of speed. -- Added to its original speed of 15 m/s, it'll then be moving at (15 + 38.5) = 53.5 m/s . There's no information in the question regarding the direction of motion at any time, so there's no way to say anything about velocity.
I also got 53.5 m :) final velocity = initial velocity + acceleration x time