Question 459600
When you are working with real numbers, the main things that will limit your domain are (1) taking the square root of a negative number, and (2) division by zero.
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In your problem, 4x + 20 is in the denominator, so we want to avoid division by zero. Set this expression equal to zero to find the value of x.
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4x + 20 = 0
4x = -20
x = -5

We see that x cannot equal -5 because we would be dividing by zero. Your domain is all real numbers except -5. 

Note: I'm still learning to use the formula rendering software on this website. As soon as I figure out how to write out this domain symbolically, I'll edit the solution. If you need the answer before then, post a question asking how to represent this domain mathematically. 

Hope this helps!