Step 1: Concept
Complete the squares for $x$ and $y$ terms.
Step 2: Meaning
$4(x^2+6x+9) + 9(y^2-2y+1) = -9 + 36 + 9$.
$4(x+3)^2 + 9(y-1)^2 = 36$.
Step 3: Analysis
The equation is of the form $\frac{(x-h)^2}{a^2} + \frac{(y-k)^2}{b^2} = 1$, where $(h, k)$ is the centre.
Step 4: Conclusion
Centre is $(-3, 1)$.
Final Answer: (D)