We will start to add the like terms
Remember (-)(+) = (-)
[tex]6a^2+7a+9-(a+4)=6a^2+7a+9-a-4[/tex]The like terms are 7a and -a, 9 and -4
[tex]6a^2+7a+9-a-4=6a^2+(7a-a)+(9-4)[/tex][tex]6a^2+(7a-a)+(9-4)=6a^2+6a+5[/tex]The answer is
[tex]6a^2+6a+5[/tex]