What is the slope of a line -3x + 20y = 40

You have to take this into y=mx+b form. So you just solve this as a equation. First, you have to take -3x to the other side of the equation. So it would look like this: 
20y= 3x +40

Then, you have to divide the 20 on each side of the equation. So, it would look like this:
y= 3/20x +2

And then that would be your answer. All you have to do for problems like this is, you have to get "y" alone.