when you say a number is the root of an expression it means that when you plug that number into the variable of the expression it evaluates to 0.

It doesn't make sense to say 3 is a root of 2x+b=0, you would say 3 is a root of 2x+b and the =0 part is automatically assumed.

so if 3 is a root of 2x+b,

then 2(3)+b=0

b=-6

Your simple side was right this time!