Answer
$\color{blue}{y=-2x+5}$
Work Step by Step
RECALL:
The slope-intercept form of a line's equation is:
$y=mx+b$
where $m$ = slope and $b$ = y-intercept
The slope of the line is $-2$.
Substitute $-2$ for $m$ into the standard form to obtain the tentative equation:
$y=-2x+b$
To find the value of $b$, substitute the values of x and y of the given point into the tentative equation above to obtain:
$y=-2x+b
\\-1=-2(3) + b
\\-1=-6+b
\\-1+6=b
\\5=b$
Thus, the equation of the line is $\color{blue}{y=-2x+5}$.