How many nanoseconds does it take light to travel 3.50 ft in vacuum?

Answer:3.56 nanosecond

In this case, you are asked the time and given the light distance(3.5ft)
To answer this question you would need to know the velocity of light. Speed of light is 299792458m/s. Then the calculation would be:

time= distance/speed
time= 3.5 ft / (299792458m/s) x 0.3048 meter/ 1 ft= 3.56
10^(-9)