Just a question... Instead of formula and solving anything, isn't "y=px+a" just like "y=mx+b" so we know "p" is the slope. And it's tilted left, so it's negative. And a slope is the point on the vertical axis divided by the point on the horizontal axis... and if it's tilted left, then you add that it's a negative slope. So -a/b is the negative slope, and p is also the negative slope. Is that right, or would it not work to reason it like this next time? Actually messing with the formula just confused me...

Michalia, your reasoning is right. I always find it easier to work with a correct reasoning (which means you understand the principle) than memorizing formulas.