The domain means the allowed values for x. In this example, you can work your way backwards through the equation.
On the outside, there is a square root. You know that the argument for the square root has to be zero or positive (>=0), so
-4x + 20 >= 0. This simplifies to x <= 5. The domain therefore is
(-∞, 5]
If you have trouble following this, please say which part needs clarification.
