[tex]V=\frac43 \pi r^3\\\\d_1=7cm\\r_1=\frac72cm\\\\V_1=\frac43\pi * (\frac72cm)^3\\V_1=\frac43\pi*\frac{343}8cm^3\\V_1=\frac{343}6\pi cm^3[/tex]
[tex]d_2=4cm\\r_2=\frac42cm=2cm\\\\V_2=\frac43\pi * (2cm)^3\\V_2=\frac43 \pi * 8cm^3\\V_2=\frac{32}3\pi cm^3[/tex]
[tex]V_3=V_1+V_2\\V_3=\frac{343}6\pi cm^3+\frac{32}3\pi cm^3\\V_3=\frac{343}6 \pi cm^3+\frac{64}{6} \pi cm^3\\V_3=\frac{407}6 \pi cm^3[/tex]
[tex]\frac{407}6 \pi cm^3 = \frac43 \pi r^3 /*\frac34\\\frac34*\frac{407}6 cm^3=r^3\\\frac14*\frac{407}2 cm^3=r^3\\\frac{407}8 cm^3=r^3\\r=\sqrt[3]{\frac{407}8}cm=\frac{\sqrt[3]{407}}2cm\\d=2r=2*\frac{\sqrt[3]{407}}2cm=\sqrt[3]{407}cm[/tex]
