input: def linear_eq(a, b, c):
      """ This function calculates the objective function of a linear equation ax + by = c.
      It returns the result of a*x + b*y - c.
      >>> linear_eq(3, 5, 7)
      0
      >>> linear_eq(2, 4, 10)
      0
      """
output:     return a * x + b * y - c