Respostas

2014-08-23T13:44:55-03:00
 \left \{ {{a+b=10} \atop {a-b=3}} 
 \left \{ {{a=10-b} \atop {a-b=3}} 

agora substitua
a-b=3
10-b-b=3
-2b=-7
b=7/2
a=10-7/2
2a=(20-70
2a=13
2a-2b=13-7=6