Exercise 17 Page 160

To write the equation of a line perpendicular to the given equation, we first need to determine its slope.

The Perpendicular Line's Slope

Two lines are perpendicular when their slopes are negative reciprocals. This means that the product of a given slope and the slope of a line perpendicular to it will be $\text{-}1.$ For any equation written in slope-intercept form, $y=mx+b,$ we can identify its slope as the value of $m.$ Looking at the given equation, we can see that its slope is $\text{-}9.$