M= 20 because the rate of change is -3.
Let's take a look at the first few numbers in the sequence based on the given rule:

Inspecting this pattern it seems like the power

is being raised to is always one less than the number of the sequence, so if we were on the nth number in the sequence, that part of the expression would be

. We also know that we'll be multiplying whatever we get from that by 6, so we can write the full explicit rule for our sequence as

Where

is the nth number in our sequence.
Answer:
√7
Step-by-step explanation:
2/5, 0.404040 and 0.286 are all rational as they all can be expressed in fraction form (0.4040 can be expressed as 40/99 and 0.286 as 286/1000 (can simplify but I didn't)) sqrt7 is the only irrational number.