Point Xj is linear combination of rest of the other points ?

[blunderboy]

I remember in lecture 7, professor used a property

For any d+2 points, we can write

Xj as linear combination of rest of the points.

Q1. What is the proof for this property ?

Let us take an example: d = 1 and points are (5,10,7)
Q2. How do we write 7 in terms of 5 and 10 ?