Help, I need some explaining on this math question

Answer:
8 in each box
Step-by-step explanation:
Step 1: 4x+5x=72
Step 2: Add 4x and 5x which is 9x, so 9x=72
Step 3: Divide both sides by 9. 72 divided by 9 is 8. So x=8
Conclusion: There are 8 granola bars in each box.