Look carefully at the solution. This 2x term is equal to 2/2 or 1. Without this, you would have no variable to solve for. Read the problem carefully. It gives you an equilibrium amount of the product,,,, If an equilibrium amount is KNOWN.....that is where you need to pounce !!!! Re-do this problem ! I told my students this summer that this type of question will toughen them up ! If equilibrium values were given.....it would be a simple plug and chug calculation. This was a bit tougher, but worth the effort when you slay the DAT Beast.
Think 30!
Hope this helps.
Dr. Romano