Answer
Radius of the sphere is $\approx2.88ft$
Work Step by Step
According to the problem, one gallon holds about $0.1337ft^3$, so volume of the sphere which holds $750$ gallons will be:
$750\times0.1337 = 100.275ft^3$
In general we know formula of Volume of a sphere ($V$) : $V=\frac{4}{3}\pi r^3$
$100.275 = \frac{4}{3}\pi r^3$
$\pi r^3 = 75.2625$
$r^3\approx23.9388$
$r\approx2.88foot$
