1- 7a - 9b + 6a - 4b
7a + 6a = 13 a
-9b -4b = 13b R// 13a-13b
2- a + b -c -b -c +2c -a
a -a= 0
b - b= 0
-c-c+2c= 0 R// 0
3- 5x -11y -9 +20x -1 -y
5x +20x= 25x
-11y - y= -12y
-9 -1= -10 R// 25x - 12y -10
4- -6m + 8n +5 -m -n -6m -11
-6m-m-6m = -13m
8n -n = 7n
5 -11= -6 R// -13m+7n-6
5- a+b +2b -2c +3a +2c -3b
a + 3a= 4a
b + 2b= 3b -3b= 0
-2c + 2c = 0 R// 4a
