when you simplify the first site it equals -16x*2+320x
the second site equals -16x*2+320x+200
both problems are representing the time after launching the fireworks.
x y x y
9 | 1784 9 | 1584
10| 1800 10 | 1600
11| 1784 11 | 1584
so both sites meet at 10, hope this helps!