What's (- and what's 33.7y+8.4-2. and 4.2x+8.1x+1.8x-2. and x+2(x-

You're first problem is unclear as I am not sure if that t was suppose to be there or not. 

but as for the second one (33.7y+8.4-2.04y)  we would first have to combine like terms so you'd subtract 2.04y from 33.7y , which would give you ; 33.66y so now your answer would be " 33.66y + 8.4 ", 
there's nothing more you can do because they're seperate terms c: . 

Your third problem ( 4.2x+8.1x+1.8x-2.1x ) is also combining like terms. Add the first 3 terms and that'll give you 14.1x , now subtract 2.1x and that'll give you "12x" as your final answer ~

For your fourth problem x+2 ( x-y ) , you would have to do something called factoring , basically all you gotta do is distribute term by term. So, x times x would give you x^2 , x times -y would give you -xy , 2 times x is 2x , and 2 times -y is -2y , so that'll look somethng like x^2 - xy + 2x - 2x . Now the 2x's cancel out
and you are left with "x^2 - xy"  as your final answer ~