You would need to use the distance formula.
Distance = Sqrt((x2-x1)^2+(y2-y1)^2)
Plug in your numbers
Distance = Sqrt((12+4)^2+(12-18)^2)
Distance = 17.09
Where's the graph? If you can give me the graph then I'd be able to answer for you.
Answer:
22.25 ft long
if you add 10.5 with 11.75 you'll get 22.25
7 + 7 + (-1)
7 + 7 - 1
13
Hope this helps :)