The tank is in the form of cylinder
its radius is 24 cm and it height till 
volume of water intially
V= \pi r^2h=1357.168 cm^3
now four sphere of radius r' are added
so the final height is 18+7=25cm
Volume of water Finally
V'= \pi r^2h= \pi (24)^2(25)=1884.96 cm^3
So the difference between the final and intial values will give the volume of spheres
4V_{sphere}=V'-V=527.79 cm^3
V_{sphere}=131.95 cm^3
 \frac{4}{3} \pi r^3_{sphere}=131.95
r_{sphere}=5.612486 cm