In a triangle, the length of any side is greater than the difference of the lengths of the other two sides and less than the sum of the lengths of the other two sides.
34 mm - 24 mm = 10 mm
The length of side XZ must be greater than 10 mm.
24 mm + 34 mm = 58 mm
The length of side XZ must be less than 58 mm.
The only choice between 10 mm and 58 mm is 42 mm.