Answer:
radius = 11
Step-by-step explanation:
The equation of a circle in general form is
x² + y² + 2gx + 2fy + c = 0
with radius = [tex]\sqrt{g^2+f^2-c}[/tex]
x² + y² - 20x + 14y + 28 = 0 ← is in general form
with 2g = - 20 ⇒ g = - 10
2f = 14 ⇒ f = 7 and c = 28, thus
radius = [tex]\sqrt{(-10)^2+7^2-28}[/tex] = [tex]\sqrt{100+49-28}[/tex] = [tex]\sqrt{121}[/tex] = 11
