18x+y written using the commutative property of addition

Ineed with number 17 i don’t get it at alll

While scuba diving, rajeev dove to a depth of 12.6 feet below the surface of the water and then descended another 8.7 feet. what expression can be used to find rajeev's new position? 12.6 – 8.7 –12.6 – 8.7 –12.6 – (–8.7) 12.6 – (–8.7)

Look at this pattern ; 1,4,9, number 10000 belongs in this pattern . what’s the place of this number?