Acceleration can be defined as the change of speed in an instant of time, that is
[tex]a = \frac{v_f-v_i}{\Delta t}[/tex]
Here,
[tex]v_f =[/tex] Final velocity
[tex]v_i =[/tex] Initial velocity
[tex]\Delta t =[/tex] Change in time
From this expression we will calculate the requested values replacing the variables in each of the given terms
PART A) The values under this condition are:
[tex]v_f = 5.3m/s[/tex]
[tex]v_i = 15m/s[/tex]
[tex]\Delta t = 10.4s[/tex]
Replacing,
[tex]a = \frac{5.3m/s-(15m/s)}{10.4s}[/tex]
[tex]a = -0.93m/s^2[/tex]
PART B ) The values under this condition are:
[tex]v_f = -15m/s[/tex]
[tex]v_i = -5.3m/s[/tex]
[tex]\Delta t = 10.4[/tex]
Replacing,
[tex]a = \frac{-(15m/s)-(-5.3m/s)}{10.4s}[/tex]
[tex]a = -0.93m/s^2[/tex]
Therefore the acceleration in the second time interval is [tex]-0.93m/s^2[/tex]
PART C) The values under this condition are:
[tex]v_f = -15m/s[/tex]
[tex]v_i = 15m/s[/tex]
[tex]\Delta t = 10.4[/tex]
Replacing,
[tex]a = \frac{-15m/s-(15m/s)}{10.4s}[/tex]
[tex]a = -2.9m/s^2[/tex]