The letter x is a placeholder for a number. If we knew what x was, then we can replace it with that number. For now it's unknown.
We start off with x markers in a single box. Then 15 are given out to be left with x-15 markers. This happens exactly 3 times which is what the diagram shows
3 copies of (x-15) = 3*(x-15) = 3x- 45
Put another way, we have 3 'x's that add to 3x, and we have 3 copies of "-15" which combine to -45
So,
3x-45 = 90
3x-45+45 = 90+45 ... add 45 to both sides
3x = 135
3x/3 = 135/3 .... divide both sides by 3
x = 45
Therefore, we started off with 45 markers in each box. If we take 15 away then x-15 = 45 - 15 = 30 markers are left. Then triple this to get 3*30 = 90 which is the total amount left over. This helps confirm the answer.
Final Answer: 45 markers were originally in each box