Let \(A\left(\frac{3}{\sqrt{a}}, \sqrt{a}\right)\), \(a \gt 0\), be a fixed point in the xy-plane. The image of A in y-axis be B and the image of B in x-axis be C. If \(D(3\cos\theta, a\sin\theta)\) is a point in the fourth quadrant such that the maximum area of \(\Delta ACD\) is 12 square units, then a is equal to ____________.