input: Write a function to calculate the value of the objective function of a linear equation. The function should take three parameters representing the coefficients a, b, and c in the equation ax + by = c. If the equation is provided with a = 3, b = 5, and c = 7, the function should return the value of ax + by - c, which would be evaluated as 3*x + 5*y - 7. If x and y are not provided, assume default values of x = 0 and y = 0.
  
  def linear_eq(a, b, c):
      x = 0
      y = 0
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7) == -7
  assert linear_eq(1, 1, 1) == -1
  assert linear_eq(0, 0, 0) == 0
  assert linear_eq(2, 4, 6) == -6
  assert linear_eq(1, 2, 3) == -3
  assert linear_eq(7, 3, 5) == -5
output: def linear_eq(a, b, c):
      x = 0
      y = 0
      return a*x + b*y - c