\[4x^{2} - 15x + 9 =\]
\[= 4 \cdot \left( x - \frac{3}{4} \right)(x - 3) =\]
\[= (4x - 3)(x - 3).\]
\[D = 225 - 144 = 81\]
\[x_{1} = \frac{15 + 9}{8} = 3;\]
\[x_{2} = \frac{15 - 9}{8} = \frac{6}{8} = \frac{3}{4}.\]