Don't change the as there is no need to do this. If you don't substitute that in then you get the right answer:
Then account for the other answer where:
what two angles of elevation will enable a projectile to reach a target 16km downrange on the same level as the gun if the projetile's initial speed is 400m/sec?
i thought i did it right , but my answers were both +/- 10 degree's different then the books answers.. using the formula i provided, can someone show me what i m doing wrong??