I don't get this proof.. Explain them line by line please.

[Insertion Sort Is O(n2)]

Suppose runtime is an2 + bn + c If any of a, b, and c are less than 0 replace the constant with its absolute value

an2 + bn + c (a + b + c)n2 + (a + b + c)n + (a + b + c) 3(a + b + c)n2 for n 1 Let c = 3(a + b + c) and let n0 = 1

How this can prove insertion sort is O(n^2)?