A girl starts from rest and reaches a walking speed of 1.4 m/s in 3.0 s. She walks at this speed for 6.0 s. The girl then slows down and comes to...

We have three time intervals to consider here. In the
first, the girl starts at 0 m/s and reaches 1.4 m/s in 3 seconds. The acceleration is
given by (v - u)/t, where v is the final velocity, u is the initial velocity and t is
the time taken.

a = (1.4 - 0)/3 = 7/15
m/s^2.

In the second interval of time, the girl travels at
a constant speed of 1.4 m/s for 6 seconds. The acceleration during this interval is
0.

In the third interval, the girl slows down from 1.4 m/s
and comes to a halt in 10 seconds. The acceleration here is -0.14
m/s^2.

Over the entire trip the acceleration is 0 as the
initial velocity and the final velocity are equal.