It is always easiest (imo) to make x or y the subject in the linear equation: i.e. the first equation becomes [tex]x=3-2y[/tex] or [tex]y=\frac{3-x}{2}[/tex] and then substitute either of these values into the quadratic equation so that becomes: [tex](3-2y)^2-4y^2=-33[/tex] or if you made y the subject: [tex]x^2-4(\frac{3-x}{2})^2=-33[/tex]
Oh and remember you can't take the square root of a negative value.