Factor. \[ 36-u^{2} \]

Step 1 ：Identify the given expression as a difference of squares problem. The difference of squares formula is \(a^{2} - b^{2} = (a - b)(a + b)\).

Step 2 ：In this case, \(a = 6\) and \(b = u\).

Step 3 ：Rewrite the expression as \((6 - u)(6 + u)\).

Step 4 ：Simplify the expression to get the final answer: \(-(u - 6)(u + 6)\).

Step 5 ：Final Answer: The factored form of the expression \(36 - u^{2}\) is \(\boxed{-(u - 6)(u + 6)}\).