Question 702219
All 3 angles must add to 180, so


S+M+L = 180


Plug in L = 3S to get


S+M+3S = 180


4S+M = 180


Then plug in S = M-30 to get


4(M-30)+M = 180


4M-120+M = 180


Solve this for M. Once you have M, you can find S using S = M-30. Once you have S, you can find L using L = 3S.