\[4x^{3} + 5x^{2} - 3x + 15\]
\[x = 3b:\]
\[4 \cdot (3b)^{3} + 5 \cdot (3b)^{2} - 3 \cdot (3b) + 15 =\]
\[= 4 \cdot 27b^{3} + 5 \cdot 9b^{2} - 9b + 15 =\]
\[= 108b^{3} + 45b^{2} - 9b + 15.\]