If y(x, t) = (5.2 mm) sin[kx + (450 rad/s)t + φ] describes a wave traveling along a string, how much time does any given point on the string take to move between displacements y = +3.4 mm and y = -3.4 mm?

An em wave does not require a medium, while a mechanical wave does. 

In the conservation of mass, mass (which is the same as energy) can never be created or destroyed in the same way oil is formed from the remains of small animals and plants that died and fell to the bottom of the sea and their mass (energy) is conserved as oil.