A good metaphor for where I'm at today.
Imagine a coordinate system: 3 axes, the x axis going from side to side in front of us, with positive numbers on the right, the y axis extending behind and in front of us, with positive numbers in front of us, and these are on a horizontal plane at our feet, and the z axis travels up and down vertically, with positive numbers above the xy plane. Now imagine a line extending from behind us and over our left shoulders towards the xy plane at an angle so that it travels down and to the right as it extends in front of us.
Imagine that light from above casts a shadow of this line onto the xy plane. That shadow has a slope, the change in y divided by the change in x, and it has a y intercept.
To calculate the xy slope and the y intercept of the xy shadow, imagine a point behind you and above your left shoulder. Light from above casts a shadow onto the xy plane, of that point, and that is its xy location. Now imagine another point, in front of you and to your right and a foot above the xy plane. Light from above casts its shadow, and that is its xy location. The coordinates of the first point's shadow are x1,y1, and the coordinates of the second point's shadow are x2,y2. The xy slope of the shadow on the xy plane of the line they define is y1-y2/x1-x2.
