We are not given a statement of similarity, so we cannot know for sure which sides are the corresponding sides. The best we can do is make an assumption.
If triangle RST is similar to triangle BCD, then we get
(11x - 4)/60 = 70/50
(11x - 4)/60 = 7/5
5(11x - 4) = 7 * 60
55x - 20 = 420
55x = 440
x = 8
