Identify the initial vertical velocity component.

The initial velocity $u$ is at an angle $\theta$ to the horizontal. The vertical component of the initial velocity is $u_y = u \sin \theta$.

At the maximum height, the vertical velocity $v_y$ is 0. Using the kinematic equation $v_y^2 = u_y^2 + 2a_y H$, where $a_y = -g$ (acceleration