Same function, different variables

Question

If I have a function $f$ which varies with $t$:

$f(t) = y$

and another variable $x$ which depends on $t$, is it customary to rewrite $f$ like so:

$f(x) = y$

Or should I nest the function instead, like so:

$f(t(x)) = y$

Answer 1

You should write

$f(t(x)) = y$