LK is actually the opposite side. What you've also been given is the length of the hypotenuse and an angle (20 degrees).
We know that:
sin(β)=Opposite/Hypotenuse=O/H
This is because of the famous:
SOH CAH TOA
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We can solve the problem by writing down:
sin(20)=LK/8.5
This means that:
LK = 8.5 * sin(20)
LK = 2.91 metres (to 2 decimal places)
