Double jump

hello.
 
I’m trying to do the same thing (double jump). I’ve tried to implement the code that was mentioned ( "x, y = obj:getLinearVelocity()

–and then something like obj:applyLinearImpulse(0, y - *2, obj.x, obj.y) "). But it’s not working for me.
 
Any ideas?
 
Thanks.