#### Answer

$y-2=-(x-1)$

#### Work Step by Step

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:
$y-2=-(x-1)$