can someone help me i don’t get this at all !!!! (my mouse is covering the letter “L”)

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Work Shown:
SM/MK = RM/ML
33/44 = x/40
3/4 = x/40
3*40 = 4*x ... cross multiply
120 = 4x
4x = 120
x = 120/4
x = 30
RM = 30