Which of the following would  notincrease the velocity of a sound wave in air?  

Simple. We know the relation v=fλv=fλ, where vv is velocity, ff is frequency and λλis wavelength.

For velocity to be constant, we get the relation f∝1λf∝1λ. This means that you can increase or decrease the frequency, and the wavelength will change appropriately so that the velocity remains constant.

Thus, only frequency/wavelength changes will not change the velocity of a wave.

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.

increase masses, decrease separation