In a rhombus, all sides are of equal length. This means that the lengths of segments \( LA \) and \( LK \) must be equal:
\[ LA = LK \]
Given that:
\[ LA = -3x + 4 \] \[ LK = -5x \]
We need to set these two expressions equal to each other:
\[ -3x + 4 = -5x \]
Now, let's solve for \( x \):
\[ 4 = -5x + 3x \] \[ 4 = -2x \] \[ x = -2 \]
Now, we can substitute \( x = -2 \) back into either expression to find the length of \( LK \):
Using \( LK \):
\[ LK = -5(-2) = 10 \]
Thus, for \( LMKF \) to be a rhombus, the length of \( LK \) must be:
\[ \boxed{10} \]