First you must combine (aka sum) like terms.
Like terms are numbers that have matching variables OR are numbers with out variables. In this case the like terms are 2x and x (they both have the variable x); -9 and 3 (both numbers without variables)
Now you may be asking, how will x get to the left side of the equation, and how will -9 get to the right side of the equation? Let's first focus on getting x to the left side of the equation:
To bring x to the left side by subtracting x to both sides (what you do on one side you must do to the other). Since x is being added on the right side, subtraction (the opposite of addition) will cancel it out (make it zero) from the right side and bring it over to the left side.
2x - x - 9 = x - x + 3
x - 9 = 0 + 3
x - 9 = 3
Now lets bring -9 to the right side:
To do this add 9 to both sides (what you do on one side you must do to the other). Since 9 is negative on the left side, addition (the opposite of subtraction/negative) will cancel it out (make it zero) from the left side and bring it over to the right side.
x - 9 + 9 = 3 + 9
x + 0 = 12
x = 12
Hope this helped!
~Just a girl in love with Shawn Mendes