Answer
$(a,c)$
Work Step by Step
Let $(m,n)$ be the missing endpoint. Using $\left(\dfrac{x_1+x_2}{2},\dfrac{y_1+y_2}{2} \right)$ or the Midpoint Formula, then the coordinates of the missing endpoint, given that the midpoint is $\left(
\dfrac{a+b}{2}, \dfrac{c+d}{2}
\right)$ and the other endpoint is $(
b,d
)$ are
\begin{array}{l}\require{cancel}
\dfrac{m+b}{2}=\dfrac{a+b}{2}
\\\text{AND}\\
\dfrac{n+d}{2}=\dfrac{c+d}{2}
.\end{array}
Solving these equations separately results to
\begin{array}{l}\require{cancel}
m+b=a+b
\\\\
m=a+b-b
\\\\
m=a
\\\text{AND}\\
n+d=c+d
\\\\
n=c+d-d
\\\\
n=c
.\end{array}
Hence, the missing endpoint is $
(a,c)
.$
