Solving complex quadratic equations which have $gf(x)$

The question is as follows:

The question

I have already substituted to get $(x^2+3x)^2-4(x^2+3x)=0$ which when expanded and simplified, becomes: $$x^4+6x^3+5x^2-12x=0$$

How do I go forward from here to get the values of x?