We can assume that the Apollo 10 spacecraft traveled at a constant speed and there is no any other force.
So, then we can use s = ut formula. (Where s is the distance, u is the speed/velocity and t is the time taken).
Before apply the formula make sure that all units are in same measures. If not those units should be converted into one measure.
Here distance is 33.9 million miles. Speed is given as 39,000 km/h. Use have to take those values into one measure. So let's convert 33.9 million miles into km.
1 mile = 1.60934 km
33.9 million mile = 1.60934 x 33.9 x 10⁶ km
= 54.56 x 10⁶ km
By applying the formula,
54.56 x 10⁶ km = (39,000 km/h) x t
t = 1398.97 h
day = 24 h
t = (1398.97 / 24) days
t = 58.29 days
Hence, Apollo spacecraft would take 58.29 days to reach Mars.