What is asked here is that you isolate y so that the equation takes the form of y = ..., where ... will be something that contains a, b and c but not y. So how do we get there? By applying some standard permutations to equations like so:
aby - b = c
First, we bring the -b term to the right hand side by adding b left and right:
aby -b+b = c+b
The -b and +b cancel out, so we get:
aby = c + b
Then, we divide left and right hand side by ab:
aby/ab = (c+b)/ab
Again, the ab/ab on the left cancels out (it is 1), so we get:
y = (c+b)/ab
And we're done!
So you have to know that it is allowed to add or subtract something (anything) to/from the left and right hand side of an equation. Likewise, you have to know that it is allowed to multiply or divide by something, as long as it isn't 0.