Fergus is amusing himself by tying knots in a rope. He ties one knot in the rope then measures its length: 87cm. He's bored so he keeps tying knots. When he has 4 identical knots tied in the rope, its length is 75cm. How long will the rope be when it has 6 knots? What was the original length without knots? If y is the length of the rope and x is the number of knots, write a linear function( equation) for this situation. What is the maximum value of x for which this function will be valid?, explain.