To solve the equation \( x^2 + 54x = -729 \) using the perfect square trinomial pattern, we first move all terms to one side of the equation:
\[ x^2 + 54x + 729 = 0. \]
Next, we want to complete the square. For the quadratic expression \( x^2 + 54x \), we take half of the coefficient of \( x \) (which is 54), square it, and add it to both sides of the equation.
- Half of 54 is \( \frac{54}{2} = 27 \).
- Squaring it gives \( 27^2 = 729 \).
Thus, we can rewrite the equation as:
\[ (x + 27)^2 = 0. \]
Now we can solve for \( x \):
- Take the square root of both sides:
\[ x + 27 = 0. \]
- Subtract 27 from both sides:
\[ x = -27. \]
So the solution to the equation is:
\[ x = -27. \]
The final answer is \( \boxed{-27} \).