Draw the $x-t$ graphs which represent positive,negative,and zero velocity.

(N/A) In graph $(a)$,the position $x$ increases with respect to time $t$,which means the slope of the graph is positive. Since velocity $v = \frac{dx}{dt}$ is the slope of the $x-t$ graph,the velocity is positive.
In graph $(b)$,the position $x$ decreases with respect to time $t$,which means the slope of the graph is negative. Therefore,the velocity is negative.
In graph $(c)$,the position $x$ remains constant with respect to time $t$,which means the slope of the graph is zero. Therefore,the velocity is zero.