Let's replace every copy of x with 7 and simplify.
[tex]\sqrt{5x+1}+9 = 3\\\\\sqrt{5*7+1}+9 = 3\\\\\sqrt{35+1}+9 = 3\\\\\sqrt{36}+9 = 3\\\\6+9 = 3 \ \text{ .... note the 6 on the left isn't negative}\\\\15 = 3\\\\[/tex]
Which is clearly false. This is probably the set of steps you followed to get "no solution".
Why is there no solution? Well let's subtract 9 from both sides to isolate the square root
[tex]\sqrt{5x+1}+9 = 3\\\\\sqrt{5x+1}+9-9 = 3-9\\\\\sqrt{5x+1} = -6\\\\[/tex]
Recall that the result of a square root operation is never negative. The range of [tex]y = \sqrt{x}[/tex] and [tex]y = \sqrt{5x+1}[/tex] is the set of nonnegative numbers. There is no way we can have the left hand side result in -6
If there was a negative sign out front the square root and we have this instead
[tex]-\sqrt{5x+1}+9 = 3\\\\[/tex]
then the answer would be x = 7
Unfortunately, there isn't such a negative sign, so we stick with "no solutions".
