[tex]V= L * l * h \\ =\ \textgreater \ L= \frac{V}{l*h} \\ ro= \frac{m}{V} =\ \textgreater \ V= \frac{m}{ro} \\ m=6,3kg = (6,3*1000)g=\ \textgreater \ m = 6300g \\ V= \frac{6300g}{0,7g/ cm^{3} }=\ \textgreater \ V= 9000 cm^{3} \\ L= \frac{9000 cm^{3} }{15cm*2cm} =\ \textgreater \ L=300cm[/tex]