A satellite launch rocket has a cylindrical fuel tank. The fuel tank can hold V cubic meters of fuel. If the tank measures d meter

V = pi (d/2)^2  *  h

V =  pi  (d^2  / 4)   *  h

h =  4V  /  ( pi  * d^2)       (meters)

this is how you find the volume of a cylinder:
$V=\pi r^2h$

in our case, the thing is using the diameter instead of r, which is no big deal ; thus you can write it as :  $\pi \left(\frac{1}{2}d\right)^2h=V$

we want to solve for h:

$\frac{\pi }{4}d^2h=V$
get rid of the fraction form:
$\pi hd^2=4V$
take pi d^2 to the other side:
$h=\frac{4V}{\pi d^2}$
and thats it!