If you are plotting the points/line and demonstrating the slope on a graph, then the previous answer is correct. You can set up a table.
However, like the previous answer suggests, the equation must be in slope-intercept format, which is y=mx+b. Since you have x = -4y, you have to do a little simple algebra to flip it around.
If x= -4y
Divide by -4 on both sides.
You get x/-4 = y OR y=x/-4, which is now in slope-intercept format.
The slope (m) is found in front of x and since x/-4 is the same as -1/4x, then you have a slope of -1/4.
This can be seen in the previous post's table. When y (rise) equals -1/2, x (run) = 2. Rise/run = -1/2 /2, which also equals -1/4.