Since \( JL \) is the perpendicular bisector of \( KM \), we have:
\[ KH = HM \]
Given the expressions for the lengths:
\[ KH = 6y + 3 \] \[ HM = 2y + 7 \]
Setting these two expressions equal to each other since \( KH = HM \):
\[ 6y + 3 = 2y + 7 \]
Now, we can solve for \( y \):
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Subtract \( 2y \) from both sides: \[ 4y + 3 = 7 \]
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Subtract 3 from both sides: \[ 4y = 4 \]
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Divide both sides by 4: \[ y = 1 \]
Now substitute \( y = 1 \) back into the expression for \( HM \):
\[ HM = 2y + 7 = 2(1) + 7 = 2 + 7 = 9 \]
Thus, the length of segment \( HM \) is \( \boxed{9} \) units.