Since Kristina made 2.4 dozen less than 6 times as many dozen as Heather, you can say that with the equation 6c - 2.4. Bc Liz made 1.8 dozen more than 5 times as many dozen as Heather, you can write that with the equation 5c + 1.8. You can put both equations equal to each other since Liz and Kristina made the same number, so 6c - 2.4 = 5c + 1.8. Solving for 'c', which shows you how many dozen cookies Heather made, we get 4.2
Hope this helped!
~BBGLUVER