Answer
$\color{blue}{y=-x+3}$
Work Step by Step
RECALL:
(1) The slope-intercept form of a line's equation is:
$y=mx+b$
where $m$=slope and $b$ = y-intercept.
(2) Parallel lines have the same (equal) slopes.
Line $L$ is parallel to the line $y=-x$, whose slope is $-1$.
Since $L$ is parallel to this line, $L$ also has a slope of $-1$.
Thus, the tentative equation of the line is $y=-x+b$.
To find the value of $b$, substitute the x and y values of the point $(1, 2)$ into the tentative equation above to obtain:
$y=-2x+b
\\2=-(1) + b
\\2+1 = b
\\3=b$
Therefore, the equation of line $L$ is $\color{blue}{y=-x+3}$.
