It means they want you to find the solution to the equation. i.e. they want you to find out what 'y' equals.
So if the equation is,
17 + 3y = 38
We can subtract 17 from both sides of the equation:
17 + 3y = 38
17 - 17 + 3y = 38 - 17
3y = 21
Now we can divide both sides of the equation by 3 to get 'y' on it's own:
(3y)/3 = 21/3
y = 7 