4a-[7-2(a+6)+5a]
4a-[7-2a-12+5a] :you distribute the 2 to the a and the 6
4a-[-5+3a] :you combine like terms(the 7&(-12) and the 2a&5a
4a+5-3a :you distribute the (-) to make +5 and -3a
a+5
