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.
There is no variable, and the equation is not valid.
n + 13 = 21. n = 21 - 13
-16
If you mean: 7x+21 = 105 Then the solution is: x = 12
28
3, 7, 21. 3x7 is the only equation that equals 21, and so these are the only factors of 21.
There is no variable, and the equation is not valid.
n + 13 = 21. n = 21 - 13
-16
31
21
If you mean: 7x+21 = 105 Then the solution is: x = 12
28
It is a linear equation in two dimensional space. Such an equation defines a line in 2-d
32
If the equation is 7x - 3y = 21 then (3, 0) and (0, -7).
n/2 + 3 = 21