If it's smooth plastic with a shiny looking breack/crack, then you've got a battle.. because an A.B.S polymer plastic is greasy & does not lend itself to glues, as the surfaces to be bonded will not key well to give adequate strength.
You can try 'welding' the break by drawing a hot soldering iron along the crack, but to be honest it's pretty much a lost cause. Buy a new chair.
howtomendit.com for free repair help, information and advice.
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