To find tax on something you multiply the base price by the tax percentage. So if something is 100 dollars and tax is 5 percent (or .05 in decimal form), you would add 5 dollars to the price. because 100*.05=5.
If we assume 6% as the tax then we multiply 22.50*.06 = 1.35, so you will add 1.35 to the price. That means the new price is 22.50 + 1.35 = 23.85.
It's just asking for what the tax is though so the answer is 1.35 at 6%. The question does say the tax is over 6% though, so that means the tax will only go up from there if the tax is over 6%