input: Write a function to calculate the objective function of a linear equation given the coefficients a, b, and c. The function should return the result of the expression a*x + b*y - c.
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  # Assert test cases:
  assert linear_eq(3, 5, 7) == 3*x + 5*y - 7
  assert linear_eq(1, 2, 3) == 1*x + 2*y - 3
  assert linear_eq(0, 0, 0) == 0*x + 0*y - 0
  assert linear_eq(-1, -1, -1) == -1*x - 1*y + 1
output: def linear_eq(a, b, c):
      return a*x + b*y - c