\[6y - 9y^{2} - 4 < 0\]
\[- \left( 9y^{2} - 6y + 4 \right) < 0\]
\[- \left( 9y^{2} - 6y + 1 + 3 \right) < 0\]
\[- \left( (3y - 1)^{2} + 3 \right) < 0\]
\[(3y - 1)^{2} \geq 0;\ \ 3 > 0;\ \ \]
\[\left( - (3y - 1)^{2} + 3 \right) < 0 \Longrightarrow верно.\]