Slope of s-t graph is constant. Hence, velocity of particle is constant. Further at time t = 0, displacement of the particle from the mean position is `s_(0)` or `-20 m` . Velocity of particle,
v = slope
`=(s_(0))/(t_(0))=(20)/(4)=5 ms^(-1)`
Motion of the particle is as shown in figure. At t = 0, particle is at `-20 m` and constant velocity of `5 ms^(-1)`. At `t_(0)=4s`, particle will pass through its mean position.