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Okay so I know that the answer is 12 but I don't understand how to get there. Am I right in thinking that we're going to use l hospitals rule for this? Can someone please guide me and explain to me the process of solving this problem? Thank you!

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First note that $$ \lim_{x\rightarrow-3}x^2-4=5>0 $$ and in this case it doesn't matter if the limit approaches from right or left.

In order to get infinity in the limit above, $-3$ must be a zero of $$ x^2+7x+B $$ So we have to impose $(-3)^2+7(-3)+B=0$ from which we get $B=12$.

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  • $\begingroup$ Got it, thank you so much! Enjoy your green check! $\endgroup$ – Dylan Apr 25 '14 at 0:39
  • $\begingroup$ Thank you very much Dylan! $\endgroup$ – Joe Apr 25 '14 at 0:44

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