Hi Mg,
for starters you have the slash in front of 'div' the wrong way around.
fix that and I will take another look :)
I think you also need \\ after {rll}
Plus you have (rrl } instead of {rll}
Ok this is how I wanted it to look
3x-7=2*-4
$$\begin{array}{rll}\\
3x-7&=&2*-4\\
3x-7&=&-8\\
3x-7+7&=&-8+7\\
3x&=&-1\\
\frac{3x}{3}&=&\frac{-1}{3}\\
x&=&\frac{-1}{3}\qquad or \\
x&=&-\frac{1}{3}\\
\end{array}$$
this is my coding
\begin{array}{rll}\\
3x-7&=&2*-4\\
3x-7&=&-8\\
3x-7+7&=&-8+7\\
3x&=&-1\\
\frac{3x}{3}&=&\frac{-1}{3}\\
x&=&\frac{-1}{3}\qquad or \\
x&=&-\frac{1}{3}\\
\end{array}
Here is another one to try
(y/3)+5=-9
hint: Subtract 5 from both sides, THEN multiply both sides by 3
If you want to concentrate on the maths instead of the coding you can use the maths formula display and just do it one line at the time, that will be easier 
