plz help me understand and show work for problems 19-21

The function s = f(t) gives the position ofa body moving on a coordinate line, with s in meters and t in seconds. 19) s =10t t2, 0 st 510 Find the body's displacement and average velocity for the given time interval. A) -200 m, -20 m/sec B) O m, 0 m/sec C) 200 m, 20 m/sec D) 200 m, -10 m/sec 20) s: -t3+2t2 2t,05t52 Find the body's speed and acceleration at the end of the time interval. MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question. Solve the problem. 21) At time t 20, the velocity of a body moving along the saxis is v = t2 -10t + 9. When is the body moving backward? A)05t9