To solve for y, we need to isolate it. Start by moving all of the values on the side the y is on that don't have y to the other side (kind of confusing the way I said it). By that, I mean subtract 3x from both sides:
W = 3x + 7y
W - 3x = 7y
Now, divide both sides by 7 to find the value of y:
W - 3x = 7y
(W - 3x)/7 = y
Hope this helps! :)