Question

factor.
$81u^2 - 36u + 4$

Explanation:

Step1: Identify perfect square terms

$81u^2=(9u)^2$, $4=2^2$

Step2: Check middle term validity

$2\times9u\times2=36u$, matches the middle term's absolute value

Step3: Write as perfect square trinomial

Since the middle term is negative, use $(a-b)^2=a^2-2ab+b^2$ where $a=9u$, $b=2$

Answer:

$(9u - 2)^2$