I know your problem. If you want to post say:

x+7<br/>x+8

It will turn into:

The workaround to this is to use double backslash ( \\ ). Double backslash acts similar like a p tag in html. So if you have more than two lines of equations, use double backslash at the start of every line of your equation.

Like in this example:

My code is:

\\ x+7
\\ x+8

Hopefully codecogs could make this automatic in their next update i.e. recognize br tag as a linebreak instead of rendering them to