How do i find the hypotenuse of a right angle on a graph

Since it's asking for the length of the horizontal leg, find the distance between the x-coordinates of F and M which is 7 units.

My teachers taught me that a^2 + b^2 = c^2. So what I would do is: 7^2+8^2 =c^2 then that reduces to 49+64=c^2 then I add those and get 113= c^2 then you take the square root of both so then you get : 10.6= c. That is your answer. You answer is A