8.2 + (–8.2) = 0 what property of math is this

Any greatly appreciated! you in advance. jody bought 20 shares of amazon at the close price of $121.00. she bought 20 more shares a year later at the price of $127.00. two years later she's sold all of her shares at the price of $133.00. her broker charges $50 for each transaction. after all of these transactions how much money did jody make? a.$65.00b.$5288.00c.$210.00d.$310.00

Iam trying to solve a linear equation for the shoe size of someone 84" tall taking into consideration example 1 is 69" tall with a shoe size of 9 and example 2 is 72 inches tall with a shoe size of 11. i understand that the equation should be y=mx+b and that on the graph the answer is (3,2) but i just don't understand how to create the equation to solve for 84-inches and 60-inches tall questions. after two days, i need .

You eat 8 strawberries and your friend eats 12 strawberries from a bowl. there are 20 strawberries left. which equation and solution give the original number of strawberries?