-10 + a = 6a - 7a
Rearrange the left hand side
a - 10 = 6a - 7a
Simplify 6a - 7a
a - 10 = -a
Add a on both sides
2a - 10 = 0
Add 10 on both sides
2a = 10
Divide by 2 on both sides
a = 5
The ratio of x:y:z = 20:8:11
Total unit = 20+8+11 = 39
Value of 1 unit ratio = 78/39 = 2
So, the water in container X, 20(2) = 40 liters
Your answer is 40 liters