I just started Macro Economics and im having trouble calculation graphs. The Equation of Linear Relationship is stated as such.
dependent var = ver intercept + slope * independent var

Now going up the slope my calculations are always correct. 250 = 50 + .5(400), 125 = 50 + .5(150) but going down the result of the calculations are incorrect.

If we are on a downward slope then the slope is calculated as horizontal change / vertical change rather than vertical change / horizontal change. Correct? 400 = 50 + 2(250) is incorrect. So does the vertical intercept have to change also? But if that is the case then we would need a vertical intercept of -100?