Answer
$\begin{bmatrix}
12& -2.5 \\
-2.5&0\\
\end{bmatrix} $
Work Step by Step
The expression $ax^2+bxy+cy^2=0$ is associated with the equation $ax^2+bxy+cy^2+dx+ey+f=0$. Then A is the quadratic form given by
$\begin{bmatrix}
a& b/2 \\
b/2&c\\
\end{bmatrix}
$.
Here $a=12,b=-5,c=0$; hence the matrix of the quadratic form is:
$\begin{bmatrix}
12& -2.5 \\
-2.5&0\\
\end{bmatrix} $
