Question 958332
You first want all terms of a on one side of the equality symbol.


{{{ab+(-3a)=3a+2b+c+(-3a)}}}
{{{ab+(-3a)=3a+(-3a)+2b+c}}}
{{{ab+(-3a)=0+2b+c}}}
{{{a(b+(-3))=2b+c}}}
{{{a=(2b+c)/(b+(-3a))}}}
{{{highlight(a=(2b+c)/(b-3a))}}}


Most people would be much less formal, and shorten this solution process to three steps.