This is asking you to rewrite the equation so that Y is alone on one side.
First, move the 4x to the other side:
4x - 5y - 4x = 9 - 4x -->
-5y = 9 - 4x
Divide both sides by -5 to get the Y alone:
-5y/-5 = (9-4x)/(-5) -->
y = (9-4x)/(-5)
You could stop there, but some people like to make the right side look a little simpler. dividing by a negative number looks a little messy, so we rewrite the equation in a few steps. All the below are all the same thing, just written in a different way.
y = -(9-4x) / 5 -->
y = (-9+4x) / 5 -->
y = (4x - 9) / 5