If a taxi moves with an initial velocity of +12.6 meters/second and has a constant acceleration of +3.1 meters/second2, what is its final velocity after 6.0 seconds?

'Acceleration' of 3.1 m/s² means that the taxi is speeding up, and
it's speed is 3.1 m/s faster each second than it was a second earlier.

So, after 6.0 seconds, its speed is  (6 x 3.1) = 18.6 m/s faster than
it was at the beginning of the 6 seconds.

That speed was 12.6 m/s when the 6 seconds began, so it's 

                     (12.6 + 18.6) = 31.2 m/s

at the end of the 6 seconds.

Notice that this whole discussion talks about the taxi's speed, not velocity ...
just as the question should.  There isn't a single word anywhere in the
question that refers to the taxi's direction, so we don't have a shred of
information that would make it possible to say anything about its velocity.