Lets expand the expression using the FOIL method
FOIL=First,Outer,Inner,Last
We'd start by multiplying the first numbers in each parenthesis by eachother:
(n-3)(n-5)
n*n=n²
Now the outer two values...
(n-3)(n-5)
-5n
The inner two values
(n-3)(n-5)
-3n
And the last two values
(n-3)(n-5)
-3*-5=15
Now just put all of the expressions together...
n²-5n-3n+15
simplify
n²-8n+15
Answer=n²-8n+15
