You can add 2 to both sides, first, to get:
[tex] \sqrt{2x+1} = 4[/tex]
Then, square both sides:
[tex]( \sqrt{2x+1} )^2 = 4^2[/tex]
[tex]2x+1 = 16[/tex]
Subtract 1:
[tex]2x = 15[/tex]
Solve for x:
[tex]x = \frac{15}{2} [/tex]
And, so that's your answer: x = 15/2
