The question states that the sum of two numbers is x, and they give us one number which is 12. We know that 12 + 12x cannot equal x. We also know that 12 + 12 + x = x will cancel out the variable. Then, we can eliminate B, as the questions stated that the sum of two numbers is x, and they gave us one number of x. We know that the other number will be the unknown value minus 12, because 12 + x - 12 = x will simplify into x = x, which is true.
