It's a binomial, not a trinomial.
The way I approach these is, to reduce this to (x+a)(x+b), I need to find a and b such that
a*b = 16
a+b = 8
Since a*b is positive, either both a and b are positive, or both a and b are negative.
Since a+b is positive, a and b must both be positive.
What are the factors of 16? 1 and 16, 2 and 8, 4 and 4. A quick glance reveals which two add up to 8.