How can we conclude that $q$ is indefinite?

Question

I want to prove the following criterion for definite matrices of quadratic form: $$g(h)=h^T\cdot A\cdot h, \ \ \ h\in \mathbb{R}^2, \ \ \ \ \ A=\begin{pmatrix}a & b \\ b & c\end{pmatrix}\in \mathbb{R}^{2\times 2} , \ \ \ \ \ D:=ac-b^2$$

1. If $D<0$ then $q$ is indefinite.

2. If $D>0$ and $a>0$ then $q$ is positive definite.

3. If $D>0$ and $a<0$ then $q$ is negative definite.

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We have that $$aq(h)=(ah_1+bh_2)^2+Dh_2^2$$

Consider the first one. There we have $D<0$. How can we conclude that $q$ is indefinite? Could you give me a hint?