Maths homework help? (Sin, cos, tan)

[geminii]

Hi guys, I'm having trouble with this maths problem (16.a) and I don't know where I'm going wrong. My answer was 21.1 degrees, but the answers say that it is 68.9 degrees. Please help and tell me where I've gone wrong!!
Thanks heaps!! 

[geminii]

I used sin:

Using sin,
Sin(thita) = 1.8/5
(thita) = sin(inverse) x 1.8/5
I put this into the calculator and it came up with 21.10019602.

I don't know what happened?

[pi]

theta = cos^-1 (adjacent/hypotenuse) = cos^-1 (1.8/5) = 68.9 degrees

You use cosine as they've given you the adjacent side to the angle, not the opposite side which would make sine a logical choice.

[geminii]

I don't get why you use cosine? Because the 1.8 is the opposite (of the angle) and the 5 is the hypotenuse so wouldn't you use sin?
Thanks

[pi]

Ah I didn't read the question correctly, you're completely right

[geminii]

Ahh thanks
Haha sometimes they get the answers wrong in the book
Thanks!