remember you can do anything to an equation as long as you do it to both sides
8-(3+b)=b-9
remember the distributive property
a(b+c)=ab+ac
distribute that invisible -1
-(3+b)=-1(3+b)=-1(3)-1(b)=-3-b
8-3-b=b-9
add like terms
5-b=b-9
add b to both sides
5=2b-9
add 9 to both sides
14=2b
divide both sides by 2
7=b
b=7