Answer:
3x - 4
Explanation:
"Difference" is a clue word that tells you to use subtraction. The other clue word is "times" but that means times, or multiplication and is pretty straight forward.
"Three times a number" means to multiply a number, but we don't know the number, so we use a variable (a letter). I put x but you could use n or c or almost any letter for the variable.
"the difference of ___ and ___" means to subtract those two things.
So we get 3x - 4