Answer
$(s-5t)^2$
Work Step by Step
RECALL:
$a^2 - 2ab + b^2=(a-b)^2$
The given trinomial can be written as:
$=s^2 - 2(s)(5t) + (5t)^2$
Factor the trinomial using the formula above with $a=s$ and $b=5t$ to have:
$=(s-5t)^2$