2. Right
3、5-a+3b=5-(a-3b)=5-(-3)=8
4. The divisor in algebraic expression cannot contain letters.
5,4,-1, 2, single precision, -5/7
6、A-2B=2x？ +3xy+2x- 1-2x？ -2xy-6x+4=xy-4x+3
It has nothing to do with x, that is, there is no x in the formula, so xy-4x=0 and y=4.