Mary had 5 nickels in her bank her dad gave her 4 more nickels how many nickels does Mary have now

How do you use the elimination method for this question? explain, because i really want to understand!

Find the value of the combination. 5 c 5

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