The formula you want to use here is AB+BC=AC
This gives you 15 + y + 2y + 11 = 29
You want to solve this by combining like terms, which you would get:
26+3y = 29
You want to get the variable on one side, so subtract 26 from 29, and you get 3y = 3
That means y = 1
To find BC, you take 2y + 11, and add 1 in the place of y.
The answer is BC = 13.
