In the picure below, do you now why ds is found to be (-sintheta i +cos theta j)?

In the picure below, do you now why ds is found to be (-sintheta i +cos theta j)?
I’m especially not understanding why there is a negative in front of the sin theta.
If you could help me, that owuld be great. thankyou.
Files: Screen Shot 2016-03-29 at 11.58.02 PM.png