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 that the slope \( m = 0 \) and the y-intercept \( b = 12 \), we can substitute these values into the equation:
\[ y = 0x + 12 \]
Since \( 0x \) is 0, the equation simplifies to:
\[ y = 12 \]
Thus, the equation of the line in slope-intercept form is:
\[ y = 12 \]