The slope-intercept form of a line is given by the equation:
\[ y = mx + b \]
where \( m \) is the slope and \( b \) is the y-intercept.
Given the slope \( m = -2 \) and the y-intercept \( b = 21 \), we can substitute these values into the slope-intercept form equation:
\[ y = -2x + 21 \]
Thus, the equation of the line in slope-intercept form is:
\[ y = -2x + 21 \]