When is the magnitude of a vector + b vector equal to the magnitude of a vector - b vector

Which of the following is not a part of the respiratory system? a. pharynx b. trachea c. pancreas d. larynx

The focal length of a relaxed human eye is approximately 1.7 cm. when we focus our eyes on a close up object, we can change the refractive power of the eye by about 16 diopters. (a) does the refractive power of our eyes increase or decrease by 16 diopters when we focus closely? explain. (b) calculate the focal length of the eye when we focus closely.

Could someone me find the answer to this problem.