If a+b=c and b=5 then a+5=c it is known as what property?​

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie

Typically a point in a three dimensional cartesian coordinate system is represented by which of the following answers in the picture !

Aconstruction worker needs to put a rectangular window in the side of a building he knows from measuring that the top and bottom of the window have a width of 8bfeet and the sides have a length of 15 feet he also measured one diagonal to be 17 feet what is the length of the other diagonal