Question 289246
Inverse operation of division is multiplication.

Not sure what the first question is really asking.

But using your example
{{{nx7 = 21}}}
to get 'n' alone means to get n on one side of the equal sign and everything else on the other side
{{{nx7 = 21}}}
{{{nx = 21/7}}}
{{{nx = 3}}}
{{{n = 3/x}}}