Answer;
B. 16.2 m/s
Step-by-step explanation;
Using the equation;
v = u + at; where v is the final velocity, u is the initial velocity, a is the acceleration and t is the time taken;
u = 21 m/s, a= -2.4 m/s^2 and t = 2 seconds
Therefore;
v = 21 + ((-2.4) × 2)
= 21 - 4.8
= 16.2 m/s