The equation y-9+9=-17+9 is an example of which property of equality?

This is the concept of algebra. from the given equation;
y-9+9=-17+9
The original equation was y-9=-17
The subtraction was undone by applying inverse operation of adding 9 to get;
y-9+9=-17+9
y=-8
The aim of doing this was to obtain the value of the unknown which is y