I want to typeset the determinants of the system 

```
$\begin{cases}
a_1x+ b_1y = c_1,\\
a_2x+ b_2y = c_2.
\end{cases}$

```

I tried

```
$D=\left| \begin{matrix}
   {{a}_{1}} & {{b}_{1}}  \\
   {{a}_{2}} & {{b}_{2}}     
          \end{matrix} \right|
={{a}_{1}}{{b}_{2}}-{{a}_{2}}{{b}_{1}}$

```

and 

```
$D_x=\Biggl| \begin{matrix}
   {{c}_{1}} & {{b}_{1}}  \\       
   {{c}_{2}} & {{b}_{2}}    
   \end{matrix}  \Biggr|
={{c}_{1}}{{b}_{2}}-{{c}_{2}}{{b}_{1}}$

```

I don't understand, I get 

![enter image description here](https://i.stack.imgur.com/zbqey.jpg)