Calculating the pH of a ammonium hydrogen sulfate solution
The hydrogen sulphate ion is a much stronger acid that the ammonium ion. Thus almost every hydronium ion in the solution comes from the dissociation of a hydrogen sulphate ion.

This said, the assumption


  $\ce{H3O+}$ has a concentration of $2x$ at equilibrium. $\ce{SO4^2-}$ and $\ce{NH3}$ each have a equilibrium concentrations of $x$.


is wrong.

When you solve the quadratic equation

$$K_a = \frac{[\ce{H3O+}] [\ce{SO4^{2-}}]}{[\ce{HSO4^{-}}]} = \frac{x^2}{c - x}$$

you will get a pH near your text book's value.