Not the easiest to explain, but here goes!
You basically need to start by working out the differences in distance that would have been travelled had the aircraft not slowed down:
ATC ask you to reduce speed at 2010 to cross the meridian at 2105. However, if you don't adjust your speed you will cross it at 2100. Work out how far past the meridian you would fly. In 5 minutes at 441kts you would travel (d=s x t) d=441 x 5/60= 36.75 nm
Therefore the reduction in speed you need to make is calculated by re-arranging the formula, (s=d / t).
Reduction in speed = reduction in distance / time to reduce by distance
= 36.75 / (55/60) [2010 to 2105]
= 40kts
Although you have been using the ground speed to make the calculations, you can assume that the TAS remains constant in relation to the GS, so the speed reduction, (40kts) would be true for both the GS and TAS. If the question asked for the actual TAS required, then you would simply deduct the answer of 40kts from the original TAS.
Hope this makes sense to you and is indeed the correct answer! Instructors please correct me if it's wrong!