crazy machines level 2

I cant figure out how toplace the dominoes in order to get the ball to knock the box off

after months of just sitting and thinking - you place 2 dominoes between the two standing dominoes on the top shelf . one behind the first already standing the domino effect - 1 into 2 into 3 into 4 hits the second ball and starts it rolling down to the box. talk about feeling really stupid! took me months to figure it out. gees!!
i kept trying toget the first ball to knock over the fixed dominoe. and then kept trying to move the fixed dominoe. oh well, live and learn.