With your function, the sqrt(2x + 4) bit contains the x-value. Since you can not square root a negative number, 2x+4 must be greater than or equal to 0.
2x + 4 >= 0
2x >= -4
x >= -2
Hence the domain is x >= -2, or x must be greater than or equal to -2.
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Verified answer
The domain is the possible x-values.
With your function, the sqrt(2x + 4) bit contains the x-value. Since you can not square root a negative number, 2x+4 must be greater than or equal to 0.
2x + 4 >= 0
2x >= -4
x >= -2
Hence the domain is x >= -2, or x must be greater than or equal to -2.