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By using the root formula i got $(2\pm \sqrt{8a})$ expression.

So, $ a $ cannot be negative and if $a$ is positive we get two solutions.

Therefore, $a=0$ is the answer??

Is my approach correct and is there a better way to solve this problem?

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    Your approach is correct (even if things could have been done faster just as Ahmed S. Attaalla answered) but the mistake is $\Delta=16-4(4-2a)=8a$ and the roots are given by $x_{1,2}=\frac{4\pm\sqrt{8a}}2=2\pm \sqrt{2a}$2017-02-23

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Yes ,your approach is correct but is seems you incorrectly calculated the roots. It is enough to note that we seek a unique solution to

$$(x-2)^2=2a$$

This is only possible if $a=0$.

And the corresponding solutions are $2\pm \sqrt{2a}$.