You want to do to one side the same thing you do to the other side.
In order to get the n by itself you divide the 25n by 25. But in order to not change the answer you need to do the same to the 70.
[tex] \frac{25n}{25} = \frac{70}{25} [/tex]
so n=[tex] \frac{70}{25} [/tex] which can be reduced to [tex] \frac{14}{5} [/tex]
so n=[tex] \frac{14}{5} [/tex]