A “list” isn’t usually a thing in math unless sufficient context is given. You can’t inherit definitions from other areas of math i guess and treat a list as something like an indexed set.
Anyway, for your specific question:
\frac{2 + 5 + s + t}{4} = \frac{2 + 5 + t}{3} \implies \boxed{3s - t = 7}
I think you it’s pretty evident now what the answer is.