The lines CD and EF are perpendicular with points $C(1,2)$, $D(3,-4)$, $E(-2,5)$, and $F(k,4)$. Find the value of the constant $k$.
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The OP presumably does not know about vectors or dot products. Here's the appropriate hint: What do you know about the slopes of perpendicular lines? Now can you do the problem?