We can use the Pythagorean Theorem to solve for side AB
a^2+b^2=c^2
a will be 6 and b will be 8 becuase those are the legs
6^2+8^2=c^2
36+64=c^2
100=c^2 (square root both sides)
c=10
Then we find the difference between 10 and 6 because 6 is the shortest leg
10-6=4 ft. so B
Hope this helps