True. If we multiply $z$ by a factor $k$, then $x$ will also be multiplied by the same factor $k$ and, consequently, y will also be multiplied by $k$.
Work Step by Step
$a,b,c$ are constants. $y=ax$ $x=bz$. Replace this equation in the above equation. $y=a(bz)=(ab)z=cz$ $y=cz$, where $c=ab$