Answer:
On Saturday, when 12 weeks passed
Step-by-step explanation:
Tom gets his hair cut every 3 weeks.
David gets his hair cut every 4 weeks.
Find the least common multiple of numbers 3 and 4:
This means that Tom and David will get their hair cut on the same day at 12th week (if the first week when they started has number 0).
You can think in another way:
Tom gets his hair cut every 3 weeks. He got his hair cut after 3 weeks, after 6 weeks, after 9 weeks, after 12 weeks passed.
David gets his hair cut every 4 weeks. He got his hair cut after 4 weeks, after 8 weeks, after 12 weeks passed.