To solve the equation \( (x^3)? = x^{-51} \), we want to determine what \( ? \) should be in order for the equation to hold true.
First, we can rewrite the left side of the equation to clarify the relationship:
\[ x^{3?} = x^{-51} \]
Since the bases are the same, we can set the exponents equal to each other:
\[ 3? = -51 \]
Now, to find \( ? \), we divide both sides by 3:
\[ ? = \frac{-51}{3} = -17 \]
Thus, the value of \( ? \) is:
\[ \boxed{-17} \]