So the whole length of the segment is LN.
An equation you can use to find MN is
LN - LM = MN
(x-3) - (6) = -21 + 2x
Simplify both sides of the equation.
x - 3 - 6 = -21 + 2x
x - 9 = -21 + 2x
x = -12 + 2x
-x = -12
x = 12
So to find MN, substitute x for 12 within the expression.
-21 + 2(12)
-21 + 24
=3
So... The length of MN is 3.