180 ÷ 3 = 60 (<em>RST</em>)
60 = 7x + 4 (<em>RT</em>)
56=7x (<em>S</em>)

We see the 3 step method. In this case, we're NOT going to go all the way to a 360, because this is NOT a rectangle neither a square. So, we only go 180. Anyways, 360 by 2 = 180. it's almost the same concept.
So, like we said, the 3 step method. Plug in RTS and simplify by 180. Then, we brake it down from there.