input: Write a function to calculate the objective function of a linear equation given in the form ax + by = c. Return the absolute value of the objective function. If the coefficients a or b are zero, return the absolute difference between c and the non-zero coefficient times its respective variable.
  assert linear_eq(3, 5, 7) == abs(3*x + 5*y - 7)
  assert linear_eq(0, 5, 7) == abs(5*y - 7)
  assert linear_eq(3, 0, 7) == abs(3*x - 7)
  assert linear_eq(0, 0, 7) == 0
output: def linear_eq(a, b, c):
      if a == 0 and b == 0:
          return 0
      elif a == 0:
          return abs(b * y - c)
      elif b == 0:
          return abs(a * x - c)
      else:
          return abs(a * x + b * y - c)