To simplify the expression \( 36 - (x + 5)^2 \), follow these steps:

1. Expand the squared term \((x + 5)^2\):
\[
(x + 5)^2 = x^2 + 10x + 25
\]

2. Substitute the expanded form back into the original expression:
\[
36 - (x^2 + 10x + 25)
\]

3. Distribute the negative sign through the parentheses:
\[
36 - x^2 - 10x - 25
\]

4. Combine like terms:
\[
36 - 25 - x^2 - 10x = 11 - x^2 - 10x
\]

So, the simplified form of the expression \( 36 - (x + 5)^2 \) is:
\[
11 - x^2 - 10x
\]