Time Series Panel Data

This is my first post on here, my apologies for any errors.

My data is organized by State and Year. There are hundreds of observations per State-Year that have been used to calculate a new variable x that is the same value for every observation within the state-year group. I just want a simple table that lists the first observation of X in each state-year group (or just the value of X for each state year group). The final table might look something like:

State Year X
AL 1980 ##
AL 1982 ##
.
.
WY 1980 ##
WY 1982 ##

I know this should be an easy fix, but I haven't been able to figure it out. The closest I can get gives me error messages about mixing 'by' and 'in' commands.

Using:
by State Year: list X

Gives me every the value of X for observation with the state-year...and all the values of X are the same within that state-year group.

-Will