input: Write a function that calculates the objective function of a linear equation.
  def linear_eq(x, y, a, b, c):
      return a*x + b*y - c
  assert linear_eq(1, 1, 3, 5, 7)==1
  assert linear_eq(2, 2, 3, 5, 7)==6
  assert linear_eq(0, 0, 3, 5, 7)==-7
  assert linear_eq(4, 3, 3, 5, 7)==10
output: def linear_eq(x, y, a, b, c):
      return a*x + b*y - c