3-7

Could be a couple of things. Here are the common interpretations:

• If you meant 3 − 7, then the result is -4.
• If you meant the interval from 3 to 7:
  • As real numbers: all x such that 3 ≤ x ≤ 7; the length is 4.
  • If you’re counting integers in that range: 3, 4, 5, 6, 7 → 5 integers.
• If you meant problems 3 through 7 in a set, please paste the problems and I’ll help with solutions.

Which one did you have in mind?