# Supply/Demand Shift (Find New Equilibrium)

Starting: supply curve $$y=x+10$$ and demand curve $$y = -x+30$$ with equilibrium at (10, 20).

The problem states that DEMAND shifts leftwards by 3 units and to find the new equilibrium. My professor did not specifically state the two equations, but a 1 to 1 relationship is implied, so I was able to find them.

(Meaning that for example, the original supply curve would be a line consisting of points $$(10,20), (11,21), (9,19)$$, etc. and the demand curve would have points $$(10,20), (11,19), (9,21)$$, etc. So slope of $$1$$ and $$-1$$)

With equilibrium at $$(10, 20)$$. My professor goes on to complete the problem by moving 3 units leftward from 10 on the x-axis and 3 units down from 20 on the y-axis. His new equilibrium is $$(7, 17)$$.

I didn't think that it was an accurate way to represent/find the new equilibrium. Supply/demand graphs are supposed to shift at every point. I then plugged it into a graph and found that it was incorrect.

DEMAND shifts leftwards by 3 to create a new DEMAND curve $$y=-x+27$$ with equilibrium at $$(8.5, 18.5)$$ which is not $$(7, 17)$$.

Was the professor wrong?

• What exactly do you mean by "My professor did not specifically state the two equations, but a 1 to 1 relationship is implied"? – Herr K. Feb 21 at 3:49
• @HerrK. He said that for example, the original supply curve would be a line consisting of points $(10, 20)$, $(11, 21)$, $(9, 19)$, etc. and the demand curve would have points $(10, 20)$, $(11, 19)$, $(9, 21)$, etc. So slope of 1 and -1. – econguy Feb 21 at 3:53
• Edited question to make sense – econguy Feb 21 at 4:05
• It does seem like your professor was mistaken. – Herr K. Feb 21 at 4:20