any number that divisible by 3 is also divisible by 6. Find a counterexample to show that the conjecture is false

Counterexample: 21 which is divisible by 3 but not by 6.

Step-by-step explanation:

Use for example the number 21 which is divisible by 3 rendering 7, but not divisible by 6.

You can find any number with at least a factor of "3", but no factor "2" in it, so any odd number divisible by 3 would work as counterexample.