Find the domain of the function...

f(x)=sqrt(5x-20)

domain is all valid values for x.

Dealing with real numbers, the inside of the sqrt cannot be a negative.

so

5x-20 cannot be a negative.

5x-20 is negative when

5x-20<0

5x < 20

x < 4

Domain is then x is all real numbers EXCEPT x<4

or

stated this way, domain is x >= 4