You're kidding, right? I really do hope you know that to make y the subject you just add 3 to both sides.

Perhaps you meant to ask how to make x the subject (maybe you're trying to find the inverse function). In which case:

solve(y - 3 = -1(x - 2/6), x)

does the trick. However, you really should be able to do this with out using a CAS.