\displaystyle  y=\frac{3(x-4)}{2x^3}=\frac{3x-12}{2x^3}=\frac{3x}{2x^3}-\frac{12}{2x^3}=\frac{3}{2x^2}-\frac{6}{x^3}=\frac32x^{-2}-6x^{-3} 