[tex]\sqrt{2x+1}=3\\\\The\ domain:\\2x+1\geq0\to x\geq-0.5\\\\D:x\in[-0.5;\ \infty)[/tex]
[tex]\sqrt{2x+1}=3\ \ \ \ |square\ both\ sides\\\\2x+1=3^2\\\\2x+1=9\ \ \ \ |subtract\ 1\ from\ both\ sides\\\\2x=8\ \ \ \ |divide\ both\ sides\ by\ 2\\\\x=4\in D\\\\Answer:\boxed{x=4}[/tex]
