I just usually do this trick in situations like these where if <em>x</em> equals 1, and <em>y</em> equals 2, then <em>z </em>is gonna be 3. 2 goes after 1, and 3 goes after 2. This trick can work in other counting-up situations like this, as long as the number go up, and are right next to each other.
So WX is 7y +4 and XY is 21, and when you add those lines you get the whole thing, which is 12y so write that like an equation 12y = 7y+4+21 12y = 7y+25 5y = 25 y = 5
