Explanation:
similar triangles means that all angles in one triangle are exactly the same as the angles in the other triangle.
and the lengths of all sides of one triangle have the same scaling factor with the lengths of the sides of the other triangle.
so, when we compare the "features" of the triangles, we see that C corresponds to S, B to R and D to T.
so, DC (50) corresponds to TS (70).
and that means that CB (60) corresponds to SR (11x - 4).
the scaling factor from BCD to RST is 70/50 = 7/5.
that means DC × 7/5 = TS.
and CB × 7/5 = SR :
60 × 7/5 = 11x - 4
12 × 7 = 11x - 4
84 = 11x - 4
88 = 11x
x = 8