welcome to the forum, mom....
since you felt that this thread should be moved....(and i agree).....i felt the best section for it was critique....besides, you've been kind enough to supply all the information neccessary....
motocross is exciting and fast paced so i am getting the impression you were panning with the action....or possibly, it's simply a matter of your camera's focus point was on something other than the rider and his bike......or!.....you might have been so excited for the jump you "clicked" before your camera beeped to signify that it had made focus.....
stuff to consider.....if i'm not wrong, these motocross trails have specific jumps that your riders will be making again and again.....next time, do as you did this time and set yourself at an angle to the rise that you're fairly certain your rider will be coming through the frame.....manually pre focus your camera on the top of the rise.....or even a foot or so closer to the lens.....and wait for that moment....
i also noticed that your aperture was at 3.6....a fairly shallow depth of field.....tighten that up a bit to 5.6.....and if that doesn't do it.....further to 8 or more..that ought to give you more depth for your focus to make you happier......you will probably need to bump up your iso to keep your shutter fast......
as far as the exposure is concerned, i would consider nailing the focus aspect of it first.....gain your confidence with that and then on the next shoot.....remember what your learned and start considering exposure.....a cloudier day could be your best friend.....
peeper