First, the parentheses aren’t really doing anything as it is all addition and subtraction, so:
( a - b ) + ( c - d ) = a - b + c - d
( 8n2 - 2n3 ) + ( 6n3 - 8n2 ) = 8n2 - 2n3 + 6n3 - 8n2
Now separate the n2 terms from the n3 terms:
8n2 - 2n3 + 6n3 - 8n2 = 8n2 - 8n2 + 6n3 - 2n3
I think you should be able to get it from here, but let me know if you need further assistance.