hi
The aim of equation is to have your letter on one side of thed equal sign and the numbers on the other side. to do this, you are allowed to use any mathematical operation you know in any quantities you want as long as you do it on both side in the same time.
I will make apparent every step and quantity I add, will be in bold.
X-2 = 11
To dispose of -2 on the left, I add +2 on each side, as -2 +2 = 0
X-2 +2 = 11 +2
X +0 = 11 +2
X = 11+2
X = 13
As I have now a single X on one side and a value on the other side of the equal sign, the calculus is over.