Vectors

I need help with these two physics questions, number 3 and number 9, they are very simple with vectors, but unfortunately I've not been able to understand them quite well, so I need how to get in the solution and some written explanation in order to fully understand them. 

Pay attention in the hint page. There it says how to use some formulas and derivatives formulas.

The answer is:

#3: 1.420 +- 0.008cm

#9:  I have angle theta approximately 16.3 degrees. On the third page, hint page, it has some clues to solve this question and also the number 3.