# How do you write an equation of a line with m=-3, b=0?

Use the slope-intercept form: $y = m x + b$, where $m$ is the slope, and $b$ is the y-intercept.
The equation for a line with a slope, $m$ of $- 3$, and a y-intercept, $b$, of 0.
$y = - 3 x$
The absence of a value for $b$ is understood to be a y-intercept, $b$, of 0.