t=30.0s
Vi=30.0 km/h
Vavg=45.0 km/h
Vf=42.0 km/h
x=? Which equation would I use to find x?

Use those maybe?



then



or this works too:



then

It's interesting that V avg (average velocity I assume) is greater than either the initial or final velocity. Which tells me that either the acceleration of the object is not constant (which would be highly unusual for a typical high school physics problem) or there is an error in the original post?

I didn't really put attention to that... but you're right, if the average velocity is true, then the problem cannot be solved without any more information.