84 is equal to 100% of the price, and if x is our discount 84x = 72 to find x rearrange the equation so x is alone : x = 72/84 which can be simplified to 6/7 or 0.857. multiplying this by 100 gives the percentage 85.7% which can be simplified to 86% if the answer is only looking for 2 significant figures.
To check the answer is correct you can do 84 x 6/7 = 72