Answer:
 3.25 seconds after B launches
Step-by-step explanation:
In the 0.25 seconds before A launches, B will have traveled ...
 distance = speed · time
 distance = (240 ft/s)(0.25 s) = 60 ft
The speed of A is 20 ft/s faster, so will close that 60 ft gap at the rate of 20 ft/s. It will take ...
 time = distance/speed
 time = 60 ft/(20 ft/s) = 3 s
The fireworks will be at the same height 3 seconds after A is launched. That is 3.25 seconds after B is launched.
_____
Additional comment
Their heights will be (3 s)(260 ft/s) = 780 ft = (3.25 s)(240 ft/s).
