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 $x = 3$