It's negative. If it had no correlation then they would be all over the place, with this you can see a steady decline.
X = # of perfomances
y = total amount of money
Income: y = 7500x
Expense: y = 5900x + 88000
Substitution:
7500x = 5900x + 88000
1600x = 88000
x = 55
y = 7500(55) = $412,500
55 performances are needed to break-even.
"Completing the square" is a step in the solution of quadratic equations. It can be accomplished without the guesswork or trial-and-error associated with methods like factoring, and it always leads to a solution. It is the method by which the quadratic formula is derived.