input: Write a function to calculate the objective function of a linear equation ax + by = c. Given values for a, b, and c, the function should return the result of a*x + b*y - c. If the given parameters are a = 3, b = 5, c = 7, the function should be able to calculate the objective function accordingly.
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7) == 0
  assert linear_eq(2, 4, 6) == 0
  assert linear_eq(1, 1, 1) == 0
output: def linear_eq(a, b, c):
      return a*x + b*y - c