1. additive identity - the number 0 (for the real numbers)
2. equation - a mathematical sentence whose verb is 'equal' (=)
3. multiplicative identity - the number 1 (for the real numbers)
4. open sentence - an equation that contains a variable or variables
5. symmetric property - when the sides of an equation are interchanged, an equivalent equation is formed; if a = b, then b = a.

2x = 15 + x;

2x - x = 15;

x = 15

9x - 1 = -11;

9x = -11 + 1;

9x = -10;

x = -10/9

3x = -39;

x = -39/3;

x = -13

To solve for x in the equation X + 9 = 30, follow these steps:

Step 1: Get rid of the constant term on the left side by subtracting 9 from both sides of the equation:

X + 9 - 9 = 30 - 9

Step 2: Simplify both sides of the equation:

X = 21

So, the value of x is 21.