The Black Pearl has once again sailed into Port Royal and is firing its guns at the fort. The cannons atop the fort wall have been disabled, but there remains a single cannon at sea level which must defend the town. If the gunner estimates that the ship is 480 meters horizontally from the guns and he knows that the cannon has a muzzle velocity of 87 m/s, at what angle (or angles) should the cannon be aimed so as to hit the Black Pearl? Ignore air resistance, assumeand that the cannon and Black Pearl are at the same height above sea level.

Note: Enter your answers as a single entry, a comma separated list or, if the cannonball cannot hit the ship, enter 'none'.

The cannon can be fired at ???????? degrees to hit the ship.

so i have got to the point.

a (t)= <0 , -9.8>

v(t)= < 87cos(a) , -9.8t + 87sin(a) >

p(t)= < , >

and we need the point (480,0)

so

and

therefore

but i don't know how to simplify it any further to get an angle?