In the expansion of \((2x^2 + \frac{a}{x})^6\), the coefficients of \(x^6\) and \(x^3\) are equal.

(a) Find the value of the non-zero constant \(a\).

(b) Find the coefficient of \(x^6\) in the expansion of \((1-x^3)(2x^2 + \frac{a}{x})^6\).