Answer:
10 you just add them and or count down
Step-by-step explanation:
x² - 12x + a = (x + b)²
x² - 12x + a = x² + 2xb + b²
-12 = 2b and a = b²
b = -6
a = b² = 36
I think i knew it but I forget
sorry!
Answer: x =6
* side length^2