Here is a graph of the circle you are referring to:
This make it easy to see that since the points -11,-4 and 3,-4 represent a line through the middle of the circle that the center of the circle falls in the exact middle of those two points. We know the y value of the center point is -4. The x value can be found by taking the mid point of -11 and -4 which is -4 so the center of the circle is located at (-4,-4)