At the bottom of page 320 Keppel shows how to test for homogeneity of regression across groups. This is equivalent to testing the interaction of a by x, where an interaction would indicate that the regression coefficient for x is significantly different across the levels of a. The a by x interaction is not statistically significant, therefore we can assume homogeneity of regression slopes. Note that we get the interaction and main effects using the ## notation in Stata 12 which expands to the main effects plus their interaction.