QGIS 3) calculate line length

what I did: I calculated each line length of 5 Pohang parts.

1. click specific stream line -> click layer -> save as line1 -> repeat untill .....line5

2. Attribute Table(속성테이블) -> field calculator -> write field name, equation

->length is added to table

3. vector tab -> analysis tool -> basic statistic -> run

-> result shows sum value.

result unit is [m] ->transform to km

*area unit is [m2]