Step-by-step solution:
1. Replace  y  with the function's output:
 y = 6 \log_5 (3x^3 - 6) 
2. Switch  x  and  y  to find the inverse:
 x = 6 \log_5 (3y^3 - 6) 
3. Isolate the logarithmic term by dividing both sides by 6:
 \frac{x}{6} = \log_5 (3y^3 - 6) 
4. Rewrite the logarithmic equation in exponential form:
 5^{\frac{x}{6}} = 3y^3 - 6 
5. Solve for  y :
 3y^3 = 5^{\frac{x}{6}} + 6 
6. Divide by 3:
 y^3 = \frac{5^{\frac{x}{6}} + 6}{3} 
7. Take the cube root of both sides to solve for  y :
 y = \sqrt[3]{\frac{5^{\frac{x}{6}} + 6}{3}} 
8. Replace  y  with  f^{-1}(x) :
 f^{-1}(x) = \sqrt[3]{\frac{5^{\frac{x}{6}} + 6}{3}}