check the picture below.
so, as you can see, the UV segment is parallel to ZW, and therefore, they're the same slope, hmmm wait just a second, what is the slope of ZW anyway?
[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&Z&(~ c &,& 0~)
% (c,d)
&W&(~ c-a &,& b~)
\end{array}
\\\\\\
% slope = m
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{b-0}{(c-a)-c}\implies \cfrac{b}{-a}[/tex]
since now we know the ZW slope, we also know what is the slope for UV, thus,
[tex]\bf \begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&U&(~ a &,& 0~)
% (c,d)
&V&(~ x &,& y~)
\end{array}
\\\\\\
% slope = m
slope = m\implies
\cfrac{\stackrel{rise}{ y_2- y_1}}{\stackrel{run}{ x_2- x_1}}\implies \cfrac{y-0}{x-a}~~=~~\stackrel{ZW's~slope}{\cfrac{b}{-a}}
\\\\\\
\begin{cases}
y-0=b\implies \boxed{y=b}\\
-----------\\
x-a=-a\implies \boxed{x=0}
\end{cases}\qquad \qquad V~(0,b)[/tex]
