I don't understand why the values on the t$-$axis are like this when I draw a graph of Z$($t$).$ I will attach the graph of Y$($t$).$ Please explain why the t$-$axis, wo$,$wm$,$wc$,$ is placed!

Since $Y()=0$ in this case. Therefore,

$Z()=0$

$($ii$)$

If then Figure $(4)$ shows the spectrum of $z(t).$

Figure $5$