input: Write a function that calculates the objective function of a linear equation given coefficients a, b, and c.
  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, 8)==2*x+4*y-8
  assert linear_eq(1, 0, 5)==1*x+0*y-5
  assert linear_eq(0, 1, 6)==0*x+1*y-6
  assert linear_eq(0, 0, 0)==0*x+0*y-0
output: def linear_eq(a, b, c):
      return lambda x, y: a*x + b*y - c