What is an equation of the line that passes through the points (3,-3) and (-3,-3)?

Find the gradient m, using:

m = \frac{y_2 - y_1}{x_2 - x_1}

Put that in:

y = mx + c

Put any point, (3, -3) or (-3, -3) in the equation and solve for c.

You'll get your equation.

Another way is to make a quick sketch and from what you already know, you should be able to give the equation of the line quickly.