input: Write a function to calculate the objective function of a linear equation given by ax + by = c. The function should return the value of the objective function, which is a*x + b*y - c.
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
      
  # Example test cases:
  result1 = linear_eq(3, 5, 7)
  print(result1)  # Expected output: -7
  
  result2 = linear_eq(4, 2, 6)
  print(result2)  # Expected output: -6
output: def linear_eq(a, b, c, x, y):
      return a*x + b*y - c