Simplify.

$$(6a^2+5a+10)-(4a^2+6a+12)$$

 

There is an invisable 1 between  the minus sign and the bracket.

Everything inside the bracket muct be multipled by -1

 

$$(6a^2+5a+10)\textcolor[rgb]{1,0,0}{-1}(4a^2+6a+12)\\\\
=(6a^2+5a+10)-4a^2-6a-12\\\\
=6a^2+5a+10-4a^2-6a-12\\\\
=6a^2-4a^2+5a-6a+10-12\\\\
=2a^2-1a-2\\\\
=2a^2-a-2\\\\$$

.