Write an expression with the five terms containing the coefficients 12 15 18 and 21

Solve for x i'll try to attach the files bc it's not letting me for some reason

On a string instrument, the length of a string varies inversely as the frequency of its vibrations. an 11-inch string has a frequency of 400 cylces per second. find the frequency of a 10-icnch string.

Joanie watched 1 out of three of a movie in the morning