The slope-intercept form of a linear equation is given by the formula:
\[ y = mx + b \]
where \( m \) is the slope and \( b \) is the y-intercept.
Given that the slope \( m \) is \(\frac{1}{3}\) and the y-intercept \( b \) is \(-21\), we can substitute these values into the formula:
\[ y = \frac{1}{3}x - 21 \]
This is the equation of the line in slope-intercept form.