input: Write a function to calculate the objective function of a linear equation given the parameters a, b, and c. It should return the value of ax + by - c. If the input parameters are a = 3, b = 5, and c = 7, the function should compute the expression for the linear equation.
  
  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, 0) == 1*x + 1*y - 0
  assert linear_eq(0, 0, 0) == 0*x + 0*y - 0
  assert linear_eq(1, 2, 3) == 1*x + 2*y - 3
output: def linear_eq(a, b, c):
      return lambda x, y: a*x + b*y - c