Hi, I'm happy to help!
First, we need to find BC. We know, because B is the midpoint of AC, that AB and BC are congruent, or the same length. We are told that AB is 3.
So, BC is 3 also.
Next, we need to find AC. AC is a combination of AB and BC, so, we just add them.
3+3=6
AC is 6.
Finally, we need to find DE. We know that AC and CE are congruent because of this sign: ≅. This means they are the same length. We can then know that CE is 6 also. To find DE we need to take the length of CD away from CE. We know that CD is 2, so now, we subtract.]
6-2=4
DE is 4.
I hope this was helpful, keep learning! :D