X = 4 is taken because sum of positive terms can not be negative.

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there is a really interesting math trick behind it.

If you see a question like this take out the number inside it (in this case is 12)

Take consecutive numbers such as:

$n(n+1)=12$

Always remember that the answer is **$n+1$**

solve it you get $n=3$ and $n+1=4$

so 4 is the answer.

Assume that there is $"-"$ instead of positive then answer would be $n$.

[If we solve by equation then it would be $x^{2}+x-12=0$ giving $x = 3$