Notation for when x=?, y=?

Is there notation for what is essentially when, for the situation, for example:

In the situation when x is 2, y is 21t (edited)

Edit:

It's to keep my math notes tidy when I'm asked to write the acceleration when time is 0; I have to write this as words though I feel it's sloppy and I'm wondering if there's notation for this situation

• ${x\choose y}={5\choose0}+t{0\choose1}$ for $t\in\mathbb{R}$?? Other than that I have no idea what you are on about. – JP McCarthy Nov 24 '11 at 22:29
• @JpMcCarthy Sorry, it's hard to explain; in the situation when x = 7, y = 21t (just another example; I hope it's clearer) – Will03uk Nov 24 '11 at 22:40
• What is your real problem? What are you hoping to accomplish here? – J. M. is a poor mathematician Nov 24 '11 at 22:51
• Edited my question – Will03uk Nov 24 '11 at 22:56
• If $\mathbf a(t)$ is the function that gives the acceleration at time $t$... then $\mathbf a(0)$ suffices, no? – J. M. is a poor mathematician Nov 24 '11 at 23:02

$$y(x)|_{x=5}=21t$$ should do.
$(x,y) = (5,21t)$?