Achild riding a bicycle at 15 meters per second
accelerates at −3.0 meters per second2 for
4.0 seconds. what is the child’s speed at the end
of this 4.0-second interval?
(1) 12 m/s (3) 3.0 m/s
(2) 27 m/s (4) 7.0 m/s

In this question, all the important information's are already provided. Based on those information's the answer to the question can be easily determined. Let us now note down all the information's given in the question.
Initial velocity (u) = 15 m/s
Acceleration (a) = - 3.0 m/s^2
Time (t) = 4 second.
Then
final velocity (v) = u + at
                          = 15 + [(-3.0) * 4]
                           = 15 - 12
                           = 3 m/s^2
So the final speed of the bicycle would be 3 m/s^2. So the correct option among all the options given in the question is option "3". I hope the procedure is clear to you.

It's have to be 12 or idk

The answer should be a.