If you ride quickly down a hill on a bicycle your eardrums are pushed in before they pop back. Why is this?

This is due to variation bin air pressure at the two different altitudes, so air rushes out through the eustachian tube to hit the eardrum and back when u reach the ground

resonant frequency

see the picture for the vector

the magnitude of the vector by pythagorean theorem

r = √( (-10)^2 + (3)^2 ) = √109

magnitude = √109 units

the direction angle between vector and x-axis is

tan^(-1)(3/10) = 16.699

so the direction is approximately 16.7 degrees north of west