Given
m: mass
m = 15 kg
vo: initial speed
vo = 0m/s
vf: final speed
vf = 20 m/s
h: height
h = ?
Procedure
In the higher part the energy is potential of the system. In the lower part the energy is kinetic.
[tex]\begin{gathered} U=K \\ \text{mgh}=\frac{1}{2}mv^2 \\ gh=\frac{1}{2}v^2 \\ h=\frac{1}{2}\cdot\frac{v^2}{g} \\ h=\frac{20^2}{2\cdot9.8} \\ h=20.40m \\ \end{gathered}[/tex]The height would be 20.40 m