Question 54332
Because both sides of the equation are fractions, you can either cross-multiply or multiply both sides by a common denominator.  Using the latter, multiplying both sides by 35 gives us {{{5*3a=-2*7}}}, which reduces to {{{15a=-14}}}.  Dividing both sides by 15 yields {{{a=expr(-14/15)}}}.
To check that, substitute it into the original equation:  {{{(3*(expr(-14/15)))/7=expr(-2/5)}}}, which simplifies to {{{-42/15/7=expr(-2/5)}}}.  That further simplifies to {{{expr(-42/15)*expr(1/7)=expr(-2/5)}}} and {{{expr(-6/15)=expr(-2/5)}}}, which finally reduces to {{{expr(-2/5)=expr(-2/5)}}}.  Because that is a true statement, we know our answer is correct!