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.