Question 314786
Horizontal means the slope equals zero.
Use the point-slope form of a line,
{{{y-yp=m(x-xp)}}}
{{{y-4=0(x-1)}}}
{{{y-4=0}}}
{{{highlight(y=4)}}}
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.
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{{{ drawing( 300, 300, -5, 5, -5, 5, grid(1),circle(-1,4,.15),graph( 300, 300, -5, 5, -5, 5, 4)) }}}