I tried using ratios but I failed. I need to subtract one to get the correct answer. I remember finding the change before, but I've forgotten how to. Any hints? 
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Hamza
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Just use two of the coordinates in the diagram to solve for $m$ and $b$. For example, you can make the two equations $$3 = -2m+b\\-3=m+b$$ Subtracting the second equation from the first results in $$(3-(-3))=m(-2-1)+(b-b) \\ \implies 6 = -3m \\ \implies m = \frac{-1}{2}$$
Now plug in $m$ into either of the two equations above to solve for $b$. Once you know $b$ and $m$, you can solve $y=4m+b$ for $y$.
graydad
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Why do you use 1 and 2 for y? Wouldn't it be intuitive to use -3 and -5 for y? @graydad – Hamza Mar 13 '15 at 22:54
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@Hamza corrected. Althought $m$ remains the same. Quite a coincidence – graydad Mar 13 '15 at 22:57
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haha yeah. Alright, thanks a lot! @graydad – Hamza Mar 13 '15 at 23:04