Mechanics/Projectiles/Angles/Trig. Identities?

[JamieB2]

Homework Statement

http://img213.imageshack.us/img213/1681/question1su9.png

Apologies for the scrappy diagram. My MS Paint skills aren't amazing.

Homework Equations

Personally, I'm not too sure. This question (I think) involves things that I haven't studied, I tried to do a little research into it on the Internet but it didn't help too much in the end. All I could really think of was

sec $$\theta$$ = 1/cos$$\theta$$

tan^2 $$\theta$$ + 1 = sec^2 $$\theta$$

sin^2 $$\theta$$ + cos^2 $$\theta$$ = 1

tan^2 $$\theta$$ = (1 - cos2$$\theta$$)/(1 + cos2$$\theta$$)

cos2$$\theta$$ = (2cos^2$$\theta$$ -1)

And maybe Pythagoras' theorem

The Attempt at a Solution

(Note; for ease of writing on paper, I replaced $$\alpha$$ with $$\theta$$, because, pathetic as it sounds, I don't like writing $$\alpha$$)

A bit of a mess, one of my lines of work went;

49sin^2$$\theta$$ + 49cos$$\theta$$ = 2401 = 49^2

sin^2 $$\theta$$ + cos^2 $$\theta$$ = 1

Divide all by cos^2$$\theta$$

(sin^2$$\theta$$)/(cos^2$$\theta$$) + 1 = sec^2$$\theta$$

tan^2$$\theta$$ + 1 = sec^2$$\theta$$

Which, obviously, doesn't help towards my answer.

What I'm most interested in is a kind of kick start, if I knew what kind of thing I'm supposed to do, I'd maybe be able to do the question myself, but I honestly do not know where to start.

 

[Doc Al]

This is meant as a physics problem, not a math problem. So start with the equations for projectile motion. Write expressions for the position as a function of time, treating vertical and horizontal position separately. Combine those equations to see what you can deduce about the launch angle.