So you need to use the formula of a cone which is V=pi * r^2 * h/3 you know the volume and the ratio so r/h=3/4, r=3h/4. So from the first formula

768pi=pi*(3h/4)^2*h/3

768=9h^2/16*h/3

768=9h^3/16*3

768=9h^3/48

768*48=9h^3

36864=9h^3

4096=h^3

h=16

If you don’t understand something just let me know.

Good luck!