1.

а) (123-254)-(23-354)= 123-254-23+354=123+100-23= 200;

б) -(95+105)-(398-98)=-95-105-398+98=-95-105-300= -200-300= -500;

2.

а) 7х- ((y-x)+3y)= 7x- (y-x+3y)= 7x-y+x+3y= 8x+4y;

б) (8a-11b)+(9b+5a)=8a-11b+9b+5a=13a-2b;

в) (6х+3y)-(6y-8x)= 6x+3y-6y+8x=14x-3y;

3.

(3x-5)-(7-4x)=2

3x-5-7+4x=2

7x=2+12

x=14:7

x=2;