A line passes through the point $(-9,1)$ and has a slope of -2 . Write an equation for this line.

Step 1 ：The equation of a line in slope-intercept form is given by $y=mx+b$, where $m$ is the slope and $b$ is the y-intercept.

Step 2 ：We know the slope $m$ is -2, and we know that the line passes through the point $(-9,1)$. We can substitute these values into the equation to solve for $b$.

Step 3 ：Substituting $m=-2$ and the point $(-9,1)$ into the equation gives us $1=-2(-9)+b$. Solving for $b$ gives us $b=-17$.

Step 4 ：Now that we have the value of $b$, we can substitute $m$ and $b$ back into the equation $y=mx+b$ to get the equation of the line.

Step 5 ：Substituting $m=-2$ and $b=-17$ into the equation gives us $y=-2x-17$.

Step 6 ：Final Answer: The equation of the line is $\boxed{{\displaystyle y=-2x-17}}$.