Question 803712
1    	a	+	9    	b 	=	12    	.............1	
Total value								
1    	a	+	2    	b 	=	-2    	.............2	
Eliminate	a

(1)-(2)

9b-2b=12-(-2)

7b=14

/7
b=2

plug b in (2)

a+2b=-2
a+2*2=-2
a+4=-2
a=-6					

It has only one solution