Hi Habeeba,
Not sure I understand your question but I will make an assumption that you are asking for two pieces where the shorter pieced is 3/7 of the longer piece
let x = the longer piece
(3/7)x= shorter piece
the two pieces combined equal .5m
x + (3/7)x = .5
10/7 * x = .5
10x = 3.5
x = .35m
(3/7)(.35m) = .15m
Hope this helps