#### Answer

$\frac{-b}{a}$

#### Work Step by Step

As we know that the slopes of the parallel lines are equal.
Slope of the line containing (a,0) and (0,b)
m = $\frac{y2-y1}{x2-x1}$ = $\frac{b-0}{0-a}$
=$\frac{-b}{a}$
Therefore the slope of the line that is parallel to the line joining(a,0) and(0,b) is $\frac{-b}{a}$