Question 830615
Do just like for an equation.  The distinction is for when multiplying by a negative number.


{{{21>3(a-7)+9}}}
{{{21>3a-21+9}}}
{{{21>3a-12}}}
{{{21+12>3a}}}
{{{33>3a}}}
{{{11>a}}}



You indicate confusion on {{{-21+9}}}.  
Starting with the expression which holds it,
{{{3a-21+9}}}
{{{3a+(-21)+9}}}
Remember how to represent that {{{(-21)+9}}} on a number line!
{{{9-21=-21+9=-12}}}