Answer:
there is no one step equation becuz
x+9 = 21
take 9 to the other side becos both are same variables then
21-9 = x
hence x= 12
next x=11 question can't be answered becuz this is not a complete question
2x+9 = 21
21-9 = 2x
12=2x
x=
12
-----
2
hwnce 12/2 = 6 therefore x= 6