What property is 7 + (4+6) = 7+n

This looks like "half" a property. I'm assuming you mean something that looks like this:

IF a + b = a + c, THEN b = c. In the expression in your problem, the role of a is being played by 7. The role of b is played by (4 + 6), and the role of c is played by n.

The conclusion would be that 4 + 6 = n.

But you asked about what the property is called. The likely name is the subtraction property of equality. How did we go from a + b = a + c to the conclusion b = c? We subtracted a from both sides of the equation.