The spring scale initially shows a weight of 3.0N for the holder and the eight identical discs. When six discs are removed the weight goes down to 1.2 N. The change in weight was 3.0N - 1.2N = 1.8 N. The removal of six identical discs reduced the weight by 1.8N, so that's the total weight of the six discs that were removed. The weight of each disc is the 1.8N/6 = 0.3 N.
From looking at your work it appears that your error was neglecting the weight of the disc holder. At one point you seem to be indicating that 3N is the mass of 8 discs, and at another point you indicate that 1.2 N is the mass of the remaining two discs. The weights indicated both before and after discs were removed include the weight of the disc holder, which isn't known.