I don't understand the technique that you are attempting to use. The first equation has two roots - call them x1 and x2. You can deternine them using the quadratic equation. You can then solve for 'a' by setting the second equation equal to zero and rearranging to get:
a = -x^3+3x^2+1
You get a different value for 'a' when you replace x by either x1 or x2. There is no one value of 'a' that results in the second equation having the same two rots as the first.