An airplane flying horizontally at a height h and speed j is to be shot down by a gun firing a shell with initial muzzle velocity v at an angle θ with the horizontal. Find the condition for the shell

Let the origin be at the gun's position.
The airplane flies at a constant height $h$ and constant speed $j$.
The shell is fired with an initial muzzle velocity $v$ at an angle $\theta$ with the horizontal.
Let $t$ be the time from firing until the shell hits the airplane.

The horizontal distance covered by the airplane in time $t$ is $x_A = j t$.
The horizontal component of the shell's initial velocity is $v_x = v \cos\theta$.
The horizontal distance covered by the shell in time $t$ is $x_S