How do you find the missing length of a triangle?

You use the Pythagoras Theorem if it is a right-angled triangle. a squared + b squared = h (longest side, diagonal) squared, then square root h to find the longest side.

if it is not a right angled triangle, then use the Sine or Cosine rule.

Sine rule for: two angles and any one side or two sides and an angle that is not in between the sides.

Cosine rule for: all three sides (but then you do not need to find a missing side) or two sides and an angle that is in between.