input: Write a function to calculate the objective function of a linear equation. Given the equation ax + by = c, the function should return the result of ax + by - c. If the function is called with a = 3, b = 5, and c = 7, it should correctly calculate the objective function.
  def linear_eq(a, b, c):
      return a*x + b*y - c
  assert linear_eq(3, 5, 7)==3*x + 5*y - 7
  assert linear_eq(2, 4, 6)==2*x + 4*y - 6
  assert linear_eq(1, 1, 1)==1*x + 1*y - 1
output: def linear_eq(a, b, c):
      return a*x + b*y - c