Sure you do !
I'll tell you what it is, and why, and you'll slap your forehead
and call yourself names.
The 'standard' form of an equation that shows the slope and intercept is
y = (slope) x + (intercept)
What if the slope is zero ?
Then the equation is y = (0) x + intercept
or
y = intercept.
Does that look anything like y = -3 ?