Does anyone know why x is normally used for an unknown number?

Rob spent 25%, percent more time on his research project than he had planned. he spent an extra h hours on the project. which of the following expressions could represent the number of hours rob actually spent on the project? two answers

If y varies directly as x2 and y=6 when x=6, find the constant of variation.

Can you me find the missing length. i attached an image.