Why is the following true?

$3n^2-100n+6$ is big $O$ of $n^2$

This has been demonstrated to be true when $c$ is $4$ and $n$ is $10$.

$3\*100-1000+6 = -694 = 694$ is the absolute value
is a big $O$ of
$4\*100 = 400$

It looks like $n^2$ is a lot less. What am I doing wrong?