You can make lots of equations that have the solution 21. Here is one:
x = 21
If you think that is too trivial, add, subtract, multiply, or divide the same number to or from both sides.
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There is no variable, and the equation is not valid.
n + 13 = 21. n = 21 - 13
-16
28
If you mean: 7x+21 = 105 Then the solution is: x = 12