Question 882941
Vertical asymptotes occur when there are discontinuities. 
In this case, when the denominator goes to zero since division by zero is undefined.
{{{x-4=0}}}
{{{x=4}}}
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{{{drawing(300,300,-10,10,-10,10,blue(line(4,-20,4,20)),graph(300,300,-10,10,-10,10,(x+8)/(x-4)))}}}