Question 153580
You were correct until the last step, where you lost your denominator.
{{{n=3/5}}}

{{{3n^2-5=3(3/5)^2-5}}}

{{{3n^2-5=3(9/25)-5}}}

{{{3n^2-5=27/25-5}}}

{{{3n^2-5=27/25-125/25}}}

{{{3n^2-5=(27-125)/25}}}

{{{3n^2-5=-98/25)}}}