Premises: $($G$->$H$)->$J

Conclusion: H$->$J

Proof attempt:

$1.($G$->$H$)->$J

$|2.$ H Assume

$||3.$ ~J Assume

$|||4.$ G$->$H Assume

$|||5.$ J $1,4,$ MP

$|||6.$ ~J$$J $3,5,$ Conj

~J$$J is not a wff in the formal logical language.