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Using eq of motion....

V=u+at

V is final velocity, u is initial velocity ,a is acceleration, t is time, here it is retardation, so a changes by -a and eq is ...

V=u-at

V=5-6x3=5-18=-13m/sec , it means after the end of the interval, object will start moving in opposite direction of the initial direction with the velocity of 13m/sec, we say that velocity is Vector quantity.

Please let me know if you need any clarification. I'm always happy to answer your questions.