Find the value of c that completes the square x^2+7x+c

Please help me with how to do this, and show me. Thank you.

Lesson 9-3 for items 8-10, a square is drawn in the coordinate plane, with vertices as shown in the diagram. then the square is reflected across the x-axis. -2 4 8. the function that describes the reflection is (x, y) - a. (x, y-3). b. (x, y-6). c. (-x, y). d. (x,- y).