when you reflect on the x-axis the (x,y) change to (x,-y) so 'x' of the original point stays the same at 3 and 'y' is no longer negative giving you just 9. you original point would be (3,9) and to find the distance between them you have to start at -9 and count to 9
-9 , -8, -7, -6, -5, -4, -3 ,-2, -1, 0, 1 ,2 ,3, 4, 5, 6, 7, 8, 9,
if you count inbetween the -9 and 9 you get 17
distance = 17