We have
[tex] \left \{ {{2a=8} \atop {a+b=2}} \right. [/tex]
First we are taking first equation, and we are solving it
2a=8 /:2 divide both sides by 2
a=4
Now we can backsubstitute a=4
a+b=2
4+b=2 /-4 (subtract 4 both sides)
4+-4+b=2-4
b=-2
The final answer is
[tex] \left \{ {{a=4} \atop {b=-2}} \right. [/tex]