Ihave this for my math review & i just don't seem to get
what is (.9x+.8y)^2

Can you take the quantity [ x + y ] and square it ?

Can you take the quantity  [ x + 2y ] and square it ?

Then you should have no trouble with the square of (.9x + .8y) .

The square of any binomial (expression with 2 terms) is

(square of the first term)
+ (square of the second term)
+ 2 times (product of the terms) .