Explanation:
x = my age
x² - 21x = 100
x² - 21x - 100 = 0
the general solution for a quadratic equation is
x = (-b ± sqrt(b² - 4ac))/(2a)
in our case
a = 1
b = -21
c = -100
x = (21 ± sqrt((-21)² - 4×1×-100))/(2×1) =
= (21 ± sqrt(441 + 400))/2 =
= (21 ± sqrt(841))/2 = (21 ± 29)/2
x1 = (21 + 29)/2 = 50/2 = 25
x2 = (21 - 29)/2 = -8/2 = -4
negative age does not make any sense, so x = 25 is our solution.
the person is 25 years old.