Probability of pulling out a white ball from a pocket?

[Guest]

I'm an English teacher who is fifteen years out of practice on my maths ability, lol. A student that I'm tutoring asked me about this problem because she apparently had not been able to visit with her maths teacher today, but it's beyond what I remember how to do, and her exam is tomorrow. Would you help me with this?

"There are three white balls in a pocket, a red ball (X), and a blue ball (Y). When you take out one ball from this pocket, the odds of a white ball coming out are one-fourth, and the probability of a blue ball coming out is two-thirds. What is the value of y-x?"

Thanks in advance.