Circle $O$ is located on the coordinate plane with center at $(2,3)$.  One endpoint of a diameter is at $(-1,-1)$.  What are the coordinates of the other endpoint of this diameter?  Express your answer as an ordered pair.
To find the other endpoint of the diameter, I need to use the fact that the center of the circle is the midpoint of the diameter. That means that the coordinates of the center are the average of the coordinates of the endpoints. Let $(x,y)$ be the unknown endpoint.  Then I have the equations $2 = \frac{-1 + x}{2}$ and $3 = \frac{-1 + y}{2}$. Solving for $x$, I get $x = 2 \cdot 2 + 1 = 5$. Solving for $y$, I get $y = 2 \cdot 3 + 1 = 7$. Therefore, the other endpoint of the diameter is $(5,7)$. 