$3-6{n}^{5}-8{n}^{4}-\left( -6{n}^{4}-3n-8{n}^{5} \right)$
When there is a $-$ in front of an expression in parentheses, change the sign of each term of the expression and remove the parentheses$3-6{n}^{5}-8{n}^{4}+6{n}^{4}+3n+8{n}^{5}$
Collect like terms$3+2{n}^{5}-8{n}^{4}+6{n}^{4}+3n$
Collect like terms$3+2{n}^{5}-2{n}^{4}+3n$
Use the commutative property to reorder the terms$2{n}^{5}-2{n}^{4}+3n+3$