The answer is 6 because 2×6=12+9=21. I found my answer by first doing 2×2=4 and knowing that 4+9=13. 13 is 8 less than 21, so that is not the correct answer. Next I did 9×2=18. 18+9=27. 27 is 6 more than 21, so again that is not the correct answer. Then I did 15×2=30, I did not even have to add nine to this because it is already 9 more than 21. So, 6 is the value of x in the equation 2x+9=21.
