How to solve these equations for x and y.. equations are 

$(x-y)(x+2y)(2x+y) = 20$ 

and

$x^2+xy+y^2 = 7$

i want the METHOD not the solutions 

Do the operations to result to $$2x^3 + 3x^2y- 3xy^2 - 2y^3 - 20 = 0 \iff \\ 2(x^3 - y^3) +3xy(x-y) - 20 = 0 \iff \\ 2(x-y)(x^2 + xy + y^2) + 3xy(x-y) - 20 = 0 \iff \\ 14(x-y)+3xy(x-y) - 20 = 0$$
That simplifies the solution so keep substituting (2) into (1) to simplify the system
