What is the slope of a line that contains the points (-1, 9) And 5, 21)

It's very simple, all you need is to plug it in the slope formula.
Slope formula: (y2 - y1) / (x2 - x1)
Then, plug
(9,21) / (-1,5)
Lastly, you solve, which will give you the slope as 0.5.