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
Some parts are missing in the queston. Find attached the picture with the complete question
Answer:

Explanation:
Let's put the information in a table step-by step.
(number of remaining students)
Juniors Seniors
Condition
- Twice juniors as seniors 2(S - 15)
- 3/4 of the juniors left 1/4×2(S - 15)
- 1/3 of seniors left 2/3×(S - 15)
At the end, there were 8 more seniors than juniors:
- 2/3×(S - 15) - 1/4×2(S - 15) = 8
Now you have obtained one equation, which you can solve to find S, the number of senior students, and then the number of junior students.
Solve the equation:



- Addtion property of equalities:


- Division property of equalities:

That is the number of senior students that came out to the information meeting, but the number of students remaining to perform in the school musical is (from the table above):

Just substitute S with 153 fo find the number of students that remained to perfom in the musical:


Answer: 45
Step-by-step explanation: j/9=5. Multiply both sides by 9: j=9×5=45. So j is 45.
Step-by-step explanation:
ATQ,
Let d be the number of kilograms of dark chocolate she buys and m be the number of kilograms of milk chocolate she buys.
She needs to buy 120 kg of chocolate in total for her next order, and her recipe calls for twice the amount of dark chocolate as milk chocolate.
So,
m = 2d .....(1)
m + d = 120 ...(2)
We can also solve the above equations,
Put m = 2d in equation (2)
2d + d = 120
3d=120
d = 40
Put d = 40 in equation (1)
m = 2(40)
m = 80
Hence, she will need 40 kg of dark chocolate and 80 kg of milk chocolate.