Find the Missing Factor or Exponent(s) $\left( x ^ { 6 } \right) ^ { ? } \left( x ^ { 6 } \right) ^ { 3 } = x ^ { 42 }$

Multiple Choice

Find the missing factor or exponent(s) .
- $\left( x ^ { 6 } \right) ^ { ? } \left( x ^ { 6 } \right) ^ { 3 } = x ^ { 42 }$

A) 18
B) 24
C) 6
D) 4