Remember Factoring polynomials? Same thing.

3x^2+7x+2=0
(3x^2+6x)+(x+2)=0

Then take out the GCF.
3x(x+2)+(x+2)=0

Reduce.
3x(x+2)=0

3x=0 x+2=0

x=0 x=-2

{-2, 0}