What is the slope of a line that passes through the origin and the point (-2,1)

If it passes through the origin, you have a second point, (0, 0)

And you can find the slope using:

\frac{y_2 - y_1}{x_2 - x_1}

That is the y coordinates of a point, minus the y coordinates of the other point, divided by the x coordinate of the first point minus the x coordinate of the second point.