input: Write a function to calculate the objective function of a linear equation ax + by = c given the coefficients a, b, and c. 
  
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7)==a*x + b*y - 7
  assert linear_eq(2, 4, 8)==a*x + b*y - 8
  assert linear_eq(1, 1, 1)==a*x + b*y - 1
  assert linear_eq(0, 0, 0)==0
output: def linear_eq(a, b, c):
      return a*x + b*y - c