input: Write a python function to solve quadratic equations given coefficients a, b, and c.
  assert quadratic(1, -3, 2) == (1.0, 2.0)
  assert quadratic(1, -2, 1) == (1.0, 1.0)
  assert quadratic(1, 1, -6) == (-3.0, 2.0)
output: import math
  
  def quadratic(a, b, c):
      discriminant = b**2 - 4*a*c
      if discriminant >= 0:
          root1 = (-b + math.sqrt(discriminant)) / (2*a)
          root2 = (-b - math.sqrt(discriminant)) / (2*a)
          return (root1, root2)
      else:
          return None