You cannot. We need more information. The question must have been a bit more complex than that.
x + 3y = 6, how do I solve for y?
Not in terms of a variable standing alone, but.....
X + 3Y = 6
subtract X from both sides
3Y = 6 - X
divide both sides by 3
Y = (6 - X)/3
======Best that can be done there. Need X value to be coherent.