When writing papers on maths, when is it appropriate to use \rightarrow or \leftrightarrow, and when should one refrain from using them. For example, would this 
be appropriate use of \leftrightarrow.
EDIT:
What about this
Credit to Servaes, Peter Woolfitt and Michael, not trying to steal or anything, just using as an example. Should I make some arrows, or should I just line them up like in the picture.
Arrows such as $\iff$ and $\implies$ mean "if and only if" and "implies" and are used to show connection between statements. In the case of what you wrote you simply need equalities "$=$".
And arrows such as $\rightarrow$ have even different meanings. This one I see most often used to specify a domain and codomain of a function, like this: $f: \mathbb{R} \rightarrow \mathbb{Z}$. But it probably has other uses too.