Question 542236
We know that if the discriminant {{{b^2-4ac}}} is equal to zero, then the expression will only have one distinct root.  

This means that 
{{{12^2-4(1)(c)=0}}}
{{{144-4c=0}}}
{{{144=4c}}}
{{{c=36}}}

Therefore {{{c=36}}} will ensure that f(x) has one distinct root.