[math] \newcommand{\NA}{{\rm NA}} \newcommand{\mat}[1]{{\bf#1}} \newcommand{\exref}[1]{\ref{##1}} \newcommand{\secstoprocess}{\all} \newcommand{\NA}{{\rm NA}} \newcommand{\mathds}{\mathbb}[/math]

The price of a stock on a given trading day changes according to the distribution

[[math]] p_X = \pmatrix{ -1 & 0 & 1 & 2 \cr 1/4 & 1/2 & 1/8 & 1/8\cr}. [[/math]]

Find the distribution for the change in stock price after two (independent) trading days.