Answer
$u=(5x-8).$
$y=u^4.$
Work Step by Step
Working from the inside out:
Take $x$; multiply it by 5 and deduct 8 $\leftarrow$ "inner function."
Take $(5x-8)$; raise it to the power of $4\leftarrow$"outer function."
$u=(5x-8).$
$y=u^4.$