In a bread recipe, the amount of flour needs to be a non-negative number. Let's evaluate each option:
- -3: This is a negative number and cannot be used to specify an amount of flour.
- 4/7: This is a positive fraction, so it can be used to specify an amount of flour.
- Square root of 3: This is a positive, irrational number (approximately 1.73), which could theoretically be used, but it is not a standard way to measure ingredients in cups.
- 2 3/4: This is a mixed number that equals 2.75, which is a positive number and a common way to measure ingredient amounts.
The usable numbers for specifying how many cups of flour should be used in a bread recipe are 4/7 and 2 3/4.