\[3x - \frac{15x^{2}}{5x + 2} =\]
\[= \frac{3x(5x + 2) - 15x^{2}}{5x + 2} =\]
\[= \frac{15x^{2} + 6x - 15x^{2}}{5x + 2} = \frac{6x}{5x + 2}.\]