make y the subject of this expression p = ((x+y)/5)^1/2

To maintain the equality everything that is done to one side needs to be done also to the other. First, we square both sides to get rid of the power on the right-hand side. What we get is p^2 = (x+y)/5. Now we get rid of the fraction on the right-hand side by multiplying both sides by 5 and we obtain 5p^2 = x+y. Then, we subtract x to both sides and we get the final expression y = 5p^2 - x.