Question 410119
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Hi
Verical lines have undefined slopes:  the x-value NEVER changes
m = change(y)/change(x) (cannot divide by zero!)
{{{drawing(300,300,-10,10,-10,10 ,blue(line(6,10,6,-10))   ,grid(1),circle(6, -9,0.3),graph(300,300,-10,10,-10,10))}}}