For many time series, particularly prices in speculative markets, the random walk model has been found to

Question:

For many time series, particularly prices in speculative markets, the random walk model has been found to give a good representation of actual data. This model is written as follows:
xt = xt-1 + εt
Show that, if this model is appropriate, forecasts of xn+h, standing at time n, are given by
x`n+h = xn (h = 1, 2, 3, c)