Your answer would be 8n³ + 6n² - 4n - 20.
The horizontal method for simplifying binomials is to simply multiply the terms in the first bracket by the terms in the second bracket. Written out, this looks like:
(2n² × 4n) + (4n × 4n) + (4 × 4n) + (2n² × -5) + (4n × -5) + (4 × -5).
Then, we just need to do the operations in the brackets, which gives us:
8n³ + 16n² + 16n - 10n² - 20n - 20
And finally we collect the like terms to simplify, which means doing 16n² - 10n² = 6n², and 16n - 20n = -4n, so the final answer is 8n³ + 6n² - 4n - 20.
I hope this helps!