$7x^2 + 7x = \sqrt{1/28(4x+9)}$
$7x^2 +7x = (\sqrt{4x+9})/(2\sqrt{7})$
$7x(x+1) = (\sqrt{4x+9})/(2\sqrt{7})$
$\sqrt{4x+9} = 14\sqrt{7}x^2 +14\sqrt{7}x$
$(\sqrt{7})\sqrt{4x+9}=98x(x+1)$
$x= 5/(7\sqrt{2}) -3/7 hoặc x= -4/7 - (\sqrt{23/2})/7$