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.