Answer:
x = 15
Explanation:
Since the triangles are similar, we can say that the ratio between the respective sides of the first triangle and the 2nd is constant.
Basically, VW / ST = VU/SR = WU/TR = k, where k is a positive real number
Solving for k is simple:
k = VU/SR = 42/21 = 2
So we found out that the 1st triangle is the 2nd triangle but scaled up 2 times.
We use this to find x.
ST * 2 = VW
2x = 30
x = 15