For a number to be divisible by 9, the sum of its digits must be divisible by 9.
Here, the number is 24x, so the sum of its digits is:
2 + 4 + x = 6 + x
Now, since 24x is divisible by 9, 6 + x must also be divisible by 9.
The only single-digit value of x that makes this true is x = 3.
Therefore, the value of x is 3.