To solve the equation \((x+10)^2 - 25 = 36\), the first step should be to isolate the squared term. You can do this by adding 25 to both sides of the equation.
So, the correct first step is:
Add 25 to both sides of the equation.
This simplifies the equation to \((x+10)^2 = 61\). After this, you can take the square root of both sides.