What is the slope of the line with equation y = –3?
i have no idea how to do this.

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  ?