Drawing lines with specified lengths

Maths

The first line is 6cm. The mystery line is 3cm longer than the first line. What is the length of the mystery line?

The first line is 11cm. The mystery line is 5cm shorter than the first line. What is the length of the mystery line?

The first line is 26cm. The mystery line is 4cm shorter than the first line. What is the length of the mystery line?

How do you measure a curved line accurately?

The question said 'Measure the longest side'. What have they done wrong?