Which measurement is the longest 3.4cm or 30mm?

To determine this, it's best to first convert the lengths to the same unit, either

• 3.4 cm, 3.0 cm; OR
• 34 mm, 30 mm

You can then clearly see that 3.4 cm (which is 34 mm) is the longer of the two measurements, i.e. 3.4 cm > 30 mm