Clowns are shot from cannon at constant speed of v= 10.4 m/s. The
angle of launch may vary. D is the max distance clown may travel,
assuming landing pad is same height as cannon. The operator wants to
shoot 2 clowns from cannon, one after another and have them land on
the same pad a distance D/2 away at the same time. How can it be done?
How long should he wait to launch the second clown? Use projectile
motion to solve, with answers being D=11m and t=1.2 s to wait to
launch 2nd clown.

I think you mean the answer is 1.5 sec, not 1.2 sec.

If z is the angle above the horizontal at which a clown is fired (at
time t = 0), then the coordinates of the clown are:

x = v t cos z
y = v t sin z - 0.5 g t^2

Since D = v^2 / g, the coordinates at impact are

x = v^2 / 2g
y = 0

Plugging these into the equations above gives z = 15 and 75 degrees.

So the first clown should be fired at an angle of 75 degrees, and will
take 2.05 sec to land.  The second clown should be fired at 15
degrees, and will take 0.55 sec to land, and so should be fired 1.5
seconds after the first.