Please help important

What number should be added to both sides of the equation to complete this square? x^2-10x=7

How do you use the elimination method for this question? explain, because i really want to understand!

Since opening night, attendance at play a has increased steadily, while attendance at play b first rose and then fell. equations modeling the daily attendance y at each play are shown below, where x is the number of days since opening night. on what day(s) was the attendance the same at both plays? what was the attendance? play a: y = 8x + 191 play b: y = -x^2 + 26x + 126