800=2000tan(theta)-[19.6/cos^2^(theta)]
800 = 2000tan(t)-[19.6/cos^2^(t)]
using 1 + tan^2 = sec^2,
800 = 2000u - [19.6(1+ u^2)] , where u = tan(t)
800 = 2000u - 19.6 - 19.6u^2
19.6u^2 - 2000u + 819.6 = 0.
Using the quadratic formula (or Maple/Mathematica/Matlab etc.) to solve this, we get
u = 101.6293572, u = 0.4114591264
and u = tan(t),
therefore, t = 1.560957, t = 0.390346 radians