If there is no mintmark on your 1974 Kennedy half, that simply means it was minted at the main mint in Philadelphia. Very common -- no added value in circulated condition -- perhaps a dollar or two for a nice uncirculated one.
There's no US half dollar with those dates. The only dual-dated halves were the Bicentennial coins that carry the dates 1776-1976 If you have a copper-nickel half dollar dated 1974 it's only worth face value.
It depends very much on the coin's date. Please check the questions "What is the value of a <date> Kennedy half dollar?". E.g. if your coin is from 1969, look for "What is the value of a 1969 Kennedy half dollar?".