Answer: 54.
Step-by-step explanation: To solve this problem, we only need to substitute n=3 in the given equation and calculate, the procedure is shown as follows:
-2n(5+n-8-3n)
with n=3:
-2(3)(5+3-8-3(3))=
-6(-9)=
54
So, when n=3 the expression equals 54.