input: Write a function to calculate the objective function of a linear equation of the form ax + by = c. The function should return the value of the expression a*x + b*y - c. If the given equation is solved for x and y, the function should return 0.
  def linear_eq(a, b, c):
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7)==0
  assert linear_eq(2, 4, 8)==0
  assert linear_eq(1, 2, 3)==0
  assert linear_eq(4, 6, 10)==0
  assert linear_eq(0, 0, 0)==0
output: def linear_eq(a, b, c):
      return a*x + b*y - c