According to the meaning of the question

[(X+2)+(X+0.4)]X divided by 2= 1.6.

Solution: Answer 1

X= 5.2-0.6 under the root sign.

Answer 2

X= 5.2-0.6 under negative root sign.

Because answer 2 doesn't fit the meaning of the question.

So give it up.

So the depth of the canal under the root sign is 5.2-0.6.

Therefore, the length of the lower base is 5.2-0.2 under the radical sign.