Answer
$m=\frac{3}{7}$
Work Step by Step
A line in slope-intercept form has the equation:
$y=mx+b$ ($m=slope$, $b=y-intercept$)
We put the equation into slope-intercept form:
$3x-7y=21$
$-7y=21-3x$
$-7y=-3x+21$
$y=\frac{3}{7}x-3$
Thus the slope is $m=\frac{3}{7}$.
