"Vertically" probably refers to stacking the polynomials on top of each other and aligning them at equal powers of x. So kind of like what you might do when adding two numbers with more than 1 digit each.
For example, the sum 123 + 456 can be computed by adding digits in the same digits place:
… 1 2 3
+ 4 5 6
… 5 7 9
Then for this sum, you have
… - 3x ² - 6x + 8
+ 8x ² + 7x - 14
… 5x ² + x - 6
