Final answer:
The proportion 1/4 = 6/x can be used to find the missing side length, x, which is equal to 24.
Step-by-step explanation:
To find the missing side length, x, we can use the proportion 1/4 = 6/x. This can be simplified to x = 4 * 6 = 24. Therefore, the missing side length, x, is 24.