Final answer:
John should buy 18 feet of fence.
Step-by-step explanation:
To convert yards to feet, we need to multiply the number of yards by 3 since there are 3 feet in 1 yard. In this case, since John is building a fence that is 6 yards long, he would need to buy 6 * 3 = 18 feet of fence.
Learn more about converting yards to feet