Hello, lyyy94!
Did you make a sketch?
Points $\displaystyle D$ and $\displaystyle E$ are the endpoints of one of the sides of a square.
If the coordinates of the midpoint of $\displaystyle DE$ are (4.5, 16) and the coordinates of $\displaystyle D$ are (2, 16),
what are the coordinates of the vertices of the square? Code:

 D M E
 *     *     *
 (2,16) (4½,16) (7,16)




  +                

Since the midpoint $\displaystyle M$ is $\displaystyle 2\tfrac{1}{2}$ units to the right of $\displaystyle D,$
. . $\displaystyle E$ must be $\displaystyle 2\tfrac{1}{2}$ units to the right of $\displaystyle M.$
Hence, $\displaystyle E$ is at $\displaystyle (7,16)$
. . The side of the square is: $\displaystyle DE = 5.$
The other two vertices are either:
. . $\displaystyle \begin{array}{cccc}
\text{5 units above }D\text{ and }E\!: & (2,21),\;(7,21) \\
\text{or} \\
\text{5 units below }D\text{ and }E\!: & (2,11),\;(7,11) \end{array}$