Explanation:
guessing
volume is always unit cubed
so in this case inches cubed or in^3
so multiply all 3 numbers
carry on is 22×14x9= 2772 in^3
leather is 18×12×8 = 1728 in^3
canvas is 18×15×10 = 2700 in^3 but the depth in ge's canvas 15" in larger than the carry on checklist
so
the leather will definitely fit