Because there are two variables r and h, there's going to be infinite combinations of r and h which will yield V = 500000.
But you can solve either for 'r' or 'h' algebraically.
Firstly note that 500 litres = 500000 cm³ = V
I chose to solve for 'h' here
(so the dependent variable is h, and independent variable is r)
h = 500000 / ( Pi * r² )
Then choose any positive value of r, say
r = 100, then h = 500000 / ( Pi * 100² )
= 15.9155 cm
You can plot a graph to show the combinations of r and h which yields V = 500000