Wow. You are already at trigo in A Maths at this point of time in Sec 3. It’s probably a new concept for you, so let me know if you need more explanation.

The idea here is to identify the correct quadrant which the angle theta lies in. Since sin theta gives a negative value, theta must lie in either the third or the fourth quadrant. Since cos theta is given to be positive, theta must lie in either the first or the fourth quadrant. We are talking about the same theta, so theta must be in the fourth quadrant.

From there, we draw a quadrants diagram, correctly invoking the Pythagoras’ Theorem and placing any negative signs where applicable. We then proceed to call out cos theta and tan theta as usual.