The question is asking you to rearrange the equation so you have y on its own on one side and everything else on the other.
We have 10x + 5y = 80. So to get y on its own, we have to minus ten x from both sides: 5y = 80 - 10x.
To get y, we have to divide both sides by 5 (whatever you do to one side, you do to the other side). so y = (80 - 10x) / 5, which is y = 16 - 2x
y = 16 - 2x is your answer!
Hope I helped! If you have any more questions, feel free to comment
