Find the length of the line segment AB if it is known that BC = 9 cm, CD = 14 cm, AD = 300 mm.

AD is the length of the entire segment, which contains segments AB, BC, CD. So, to find this segment, you need to add the segments from which it consists:
AD = AB + BC + CD
Hence,
AB = AD-BC-CD
To add up all the lengths, you need them to be in the same measurement system (let us have cm). Then we translate the segment AD into cm: 300 mm = 30 cm.
AB = 30 cm – 9 cm – 14 cm = 7 cm
Answer: AB = 7 cm