Property used in the equation 9+3+1

That is the associative property.  In addition, it doesn't matter what order you add things in, so you can "associate" any grouping of additions and get the same result:

(a+b)+c = a+(b+c)

Note that this is true for multiplication as well:

(a*b)*c = a*(b*c)

there are no short passages to look at

the balance is found at the link

step-by-step explanation: