what is the slope for (-1, 5) and (3,-1)?

The slope is (difference in y_values/difference in x-values).

$${\mathtt{slope}} = {\frac{\left({\mathtt{\,-\,}}{\mathtt{1}}{\mathtt{\,-\,}}{\mathtt{5}}\right)}{\left({\mathtt{3}}{\mathtt{\,\small\textbf+\,}}{\mathtt{1}}\right)}} \Rightarrow {\mathtt{slope}} = -{\mathtt{1.5}}$$