\[3x^{3} - 5x^{2} + 2x - 15;\]
\[x = 2a:\]
\[3 \cdot (2a)^{3} - 5 \cdot (2a)^{2} + 2 \cdot 2a - 15 =\]
\[= 3 \cdot 8a^{3} - 5 \cdot 4a^{2} + 4a - 15 =\]
\[= 24a³ - 20a^{2} + 4a - 15.\]