Question 954781
Can't solve an expression, you can solve an equation. 
Do you mean {{{a^2-8a+12=0}}}
{{{(a-2)(a-6)=0}}}
Two solutions:
{{{a-2=0}}}
{{{a=2}}}
and
{{{a-6=0}}}
{{{a=6}}}
If that's not what you wanted, repost.