Answer
The four on the left hand side of the equal sign was not subtracted from both sides before both sides were squared.
Work Step by Step
$\sqrt {5x-1} +4=7$
$\sqrt {5x-1} +4-4=7-4$
$\sqrt {5x-1} =3$
$(\sqrt {5x-1})^2 =3^2$
$5x-1=9$
$5x-1+1=9+1$
$5x=10$
$5x/5=10/5$
$x=2$
