$\left[ U \right] = \frac{{[A]\,[{x^{1/2}}]}}{{[{x^2}]\, + \,[B]}}$
$⇒ [M{L^2}{T^{ - 2}}] = \frac{{[A]\,[{L^{1/2}}]}}{{[{L^2}]}}$
$\therefore\,[A] = [M{L^{7/2}}{T^{ - 2}}]$
$[AB]\, = \,[M{L^{7/2}}{T^{ - 2}}] \times [{L^2}]$ $ = [M{L^{11/2}}{T^{ - 2}}]$