## Thinking Mathematically (6th Edition)

$m=-\frac{b}{a},\text{falls}$.
Take $\left( {{x}_{1}},{{y}_{1}} \right)=\left( -a,0 \right)$and $\left( {{x}_{2}},{{y}_{2}} \right)=\left( 0,-b \right)$. Slope of the line is: $m=\frac{{{y}_{2}}-{{y}_{1}}}{{{x}_{2}}-{{x}_{1}}}$ \begin{align} & m=\frac{-b-0}{0-\left( -a \right)} \\ & =-\frac{b}{a} \end{align} Therefore, the slope of the line is $-\frac{b}{a}$. There is a vertical change of $-b$ units ($b$units down) for each horizontal change of $a$ units. The slope is negative. So, the line falls from left to right.