To solve this you need to use the formula for Chord-Chord length. You need to know it, it is ab=cd. with this, we can plug numbers in.
a=30
b=x+1
c=2x-6
d=21
So our new equation is:
30(x+1)=21(2x-6)
30x+30=42x-126
156=12x
x=13
This isn't what the question is askign for though, it wants YZ
Which is A+B or 30+x+1 or 21+13=34.
YZ=34