AC and BC are equal, so if you set -y + 23 as equal to 6y + 2, you get -y + 23 = 6y + 2. You can then subtract 2 from both sides, to get -y + 21 = 6y. Then add y to both sides, and you can get 7y = 21, which then simplifies to Y = 3. Then do -3 + 23 to find AC, to get 20.
