x is a letter which is called a variable in algebra.
It stands for a number. It can be any number, but it can be only one number at a time.
If you have just x, then it is the same as 1x.
You have 1x, meaning you have 1 time an unknown number.
Then you add to it 4x, or 4 times the same unknown number.
Then you add to it 6x, or 6 times the same unknown number.
1 + 4 + 6 = 11
That means you now have 11 times the unknown number x, or 11x
x + 4x + 6x = 11x
