Question 142577
Linear equation: y = 3x + 5
Domain: all Real Numbers
Why?  x is an independent variable that can take on any Real value
unless there is some reason in the equation which restricts values
of x. 
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Examples of restricted domains:
y = 3/(x+2)
x cannot take the value -2 because that would make the fraction 
meaningless (Note: You cannot divide by zero)
Domain: All Real Numbers except x = -2
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y = sqrt(x-3)
The x-3 cannot be negative because the square root of a negative
is not a Real Number.
So x must be greater than or equal to 3.
Domain: All Real Numbers greater than or equal to 3.
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Comment: Those examples show the primary reasons for restricting
the domain:
1st: Denominators cannot zero
2nd: You cannot take the square root of a negative.
There are other conditions that restrict the Domain of a function
but I am assuming you are dealing with fundamentals of algebra.
Cheers,
Stan H.