I can't seem to understand a thing about calculating a gradient of a straight line

Even worse I have to revise it for my test tomorrow but I can't comprehend it. I've done 6-8 hours of studying in total within the past 2/3 days on this single subject and yet just about every source I look at tells me something different. It's either "y1 - y2 / x1 - x2" or "y1 - x1 / y2 - x2", or to choose 2 random points and count the squares by the axis. Even I'm wondering if anyone could give me even a last bit of hope for this by explaining this equation ?

The gradient is (the change in y)/(the change in x) so that would be (y2-y1)/(x2-x1) and also remember that a positive gradient looks like this / and a negative gradient looks like this \. Also it's always helpful to check your work but substituting the points into the equation once you've found it!