What is 21=4x-9-x

21 = 4x -9 -x                 ---Combine 4x and -x to get 3x

21 = 3x - 9                  ---Add 9 to both sides to get rid of the -9 on the right side

+9        + 9

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30 = 3x                  ---Divide both sdies by 3 to find x!

30 ÷ 3 = 3 ÷ 3x

10 = 1x

10 = x

x = 10