a) In OA the velocity is changing with te time and it is a positive acceleration in graph. its actually non uniform motion.
b ) v = u + at
at = v - u
a = (v-u) / t
v = final velocity
u = initial velocity
t = time
in BC from graph
v = 0 m/s
u = 60 m/s
t = 80- 60 = 20 s
so a = (0-60)/20 = - 30 m / s^2
- indicates negative acceleration
c) distance = v × t
from graphfor AB
v = 60 m/s
t = 60-20 = 40 s
d = 60×40 = 2400 m