## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$y-2=-(x-1)$
The equation of a line in the point-slope form is the following: $y-y_1=m(x-x_1)$, where $m$ is the slope and the point $(x_1,y_1)$ is on the graph. In order to determine the equation of a line, we have to calculate the slope. For this, we can use the rule, that parallel lines have the same slopes. Here, a parallel line is $y=-x$. In this form, the slope can be seen as the coefficient of $x$, here $-1$ Therefore our line's slope is also $m=-1$. Therefore, using the point $(1, 2)$ and the slope $-1$, the equation can be written as: $y-2=-(x-1)$