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?