URGENT - sine graph help Can someone please answer my last question I posted. Thanks

Question:

I'm trying to graph a sine wave given different frequencies and I'm not sure how to.

Frequencies I am using: 261.625 Hz, 329.627 Hz, 391.995 Hz
I tried using the formula y(t)= sin (2πft) so the formula would be y(261.625)= sin 2π(261.625)t (using the first frequency), but it didn't work. I've seen people graph it as y= sin (261.625x)/(440) and I tried it and it works, but I don't understand how they got this formula/why to use it.

Look at the following graph of the given equation. determine whether the equation is a function. explain why or why not.

Apsychology student wishes to investigate differences in political opinions between business majors and political science majors at her college. she randomly selects 100 students from the 260 business majors and 100 students from the 180 political science majors. does this sampling plan result in a simple random sample? why or why not? no, because each group of 200 students in the sample does not have the same chance of being selected. yes, because each group of 200 students in the sample has the same chance of being selected. no, because each individual student does not have an equal chance of being selected. yes, because each individual student has the same chance of being selected.

Which equation results from adding the equations in this system? x+6y=9 and -x+2y=-15