Doesn't that just mean to solve for y?
You need to learn to write your problems out correctly. In your first post you wrote "make y the subject 4x+3y+15" which makes no sense- that is an expression, not an equation and has no "subject". Gamma assumed you mean 4x+ 3y+ 15= 0. If that was what you meant, you are lucky!
Now you write "(4,y) (2,7)" and say "you have to solve for y again..." I think you mean that (4,y) and (2,7) are points some line must pass through. The slope of such a line would be but unless you are given the slope to set that equal to, I see no equation to solve for y.