Sandra is riding a bicycle at 10.0 meters/seconds. she slows down to 2.0 meters/seconds in 10 seconds. her acceleration is. how many meeters / seconds ?

a = - 0.8 m/s2    distance =x = 60m

Explanation: vo=10m/s  , vf=2m/s , t=10s

a= (vf-vo)/t = (2-10) /10= - 0.8 m/s2 (sign means reduce your seed)

the second question is not clear . i do believe that you need the distance sandra moved (units .. meters ... not meters/second)

x=xo + vot+ at*t/2 = 0+10*10-0.8*10*10/2 = 60m

best wishes form colombia