A quadratic a function has a form of,
The first function has a term
which doesn't fit the profile of a quadratic function. The highest exponent on x inside a quadratic function can be 2, but here we have 3 so this is not a quadratic function, but rather a cubic function.
The second function fits the form of a quadratic function perfectly.
The third function is a bit tricky. While technically the third function could be considered quadratic if the leading term would be something like
and we did't even see it written out because multiplying with 0. But when we specified the form of a quadratic function, we strictly said that the number before
aka
cannot equal to zero. So the last function is not a quadratic function but rather a linear function.
Hope this helps :)