You know y will always be the output, and x will always be the input. Since you only have a rate, with a starting point of 0, you just have to worry about multiplication. They’re going up at a rate of 50, so you would multiply 50 by x. So B, y=50x.
You would want to convert miles per hour into feet per hour, then you should turn feet per hour into feet per second. Alternatively, you could turn miles per hour into miles per second, and then turn miles per second into feet per second