21
There is no variable, and the equation is not valid.
5
Zero.
n + 13 = 21. n = 21 - 13
-21
It is an equation and the value of x is 7
It is a symbolic equation which requires triangle = 21.
21
21 is the only solution of that equation.
3x + 7 = -4x + 28 implies 7x + 7 = 28
The value of x is -21.