How did destroying the city of thebes prove alexander to be more than a mere boy?

The correct answer to this open question is the following.

When Alexander destroyed the city of Thebes, he proved to be more than "a mere boy," as the Greeks considered him. But when he led the troops to victory, he showed he had the skills, abilities, and leadership of a true warrior and strategist. This made people really respect and trust him, and word of mouth spread over other regions. He burned all the houses in Thebes and sent a clear message to his enemies.

where is the picture?

william mckinley

ulysses s. grant

benjamin harrison

by the way what grade r u in k12?