Convert the given graph into V, t graph (initial velocity is u)

from ${\text{t}}_{\text{1}}\text{to}{\text{t}}_{\text{2}}$ a = const.

so u increases linearly

from ${\text{t}}_{\text{2}}\text{to}{\text{t}}_{\text{3}}$ a = 0 so v is constant

from ${\text{t}}_{\text{3}}\text{to}{\text{t}}_{\text{4}}$ a is $-\text{ve}$ and constant, so v decreases

for $\text{t}>{\text{t}}_4$ a = 0 ;  U is constant