which page of Carl Book does the equation of P8 line 2 follow from?

[Jiongcheng-Li]

which page of "A Practical Guide to Splines (revised ed., Carl De Boor 1978)" [red box] does the equation [green box] of P8 line 2 in your paper follow from? Frankly, I have difficult to find the similar Equation in Carl's Book. Could you give me a hand ?