Solve three equations with three unknowns Solve the system:
$$\begin{cases}a+b+c=6\\ab+ac+bc=11\\abc=6\end{cases}$$
The solution is:
$a=1,b=2,c=3$
How can I solve it?

For class work it is likely that the roots are integers, so I would just try them.  There are not many factorizations of $6$ and $1,2,3$ should jump out.  Then just try it and you are done.  
The routine approach is substitution.  Write the first as $a=6-b-c$ and plug that into the other two.  Solve the second for $b$ and you have one (messy) equation in $c$.  The rational root theorem will work here.
