As the question points out, the equation is indeed separable:
[tex]\dfrac{\mathrm dy}{\mathrm dx}=\dfrac{6x+9}{12y^2+16y+6}[/tex]
[tex]\implies2(6y^2+8y+3)\,\mathrm dy=3(2x+3)\,\mathrm dx[/tex]
Integrate both sides to get
[tex]2(2y^3+4y^2+3y)=3(x^2+3x)+C[/tex]
[tex]\implies F(x,y)=\underbrace{(-3x^2-9x)}_{G(x)}+\underbrace{4y^3+8y^2+6y}_{H(y)}=C[/tex]