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.