Establish a bijection between regions of ag and acylic

Question

Math 121c: Topics in Geometric Combinatorics, Spring 2012 Problems-

(a) Let G be a graph vertices {1, 2, . . . , n}. The Graphical arrangement A_G is the arrangement in Rⁿ consisting of the hyperplanes

x_i - x_j = 0, ij ∈ E(G).

Let P_G(t) be the chromatic polynomial of G, i.e. for any positive integer k, P_G(k) is the number of proper vertex colorings of G with k colors (feel free to ask me why P_G(t) is a polynomial if you don't already know why). Prove that

χA_G(t) = P_G(t).

(b) Establish a bijection between regions of A_G and acylic orientations of G, and conclude that the number of acyclic orientations of G is |P_G(-1)|.