I finally see where I went wrong. I chose down as the positive direction for y to not have to deal with negative numbers, hence the positive acceleration and positive 15m.
Where this went wrong is that the formula already factors a negative downward acceleration into it with the -(1/2)gt^2 so the downward direction has to be negative.
Thank you so much for helping me realize this.
I ended up with the right answer of 22.8 degrees.