9yx9y+yxy-8yx8y

Because an apple a day keeps the doctor away. You don't ever hear people sayin how "A CD player a day keeps the doctor away," or how "A truck a day keeps the docor away," now do you? You don't. Exactly.

If your goal is to simplify, here's how:

9yx9y+yxy-8yx8y

= 81xy² + xy² - 64xy²

81 apples + 1 apple - 64 apples = 18 apples

81xy² + xy² - 64xy² = 18xy²

From calculator:

9y*9y+y*y-8y*8y = 18*y^2

Steps:

9y*9y+y*y-8y*8y

=81y*y+y*y-64y*y

=(81-64+1)y*y

=18 y^2