Answer
$w\approx 26$ in
Work Step by Step
The ratio of the length to the width in Golden rectangle is $\frac{1+\sqrt5}{2}$. So:
$$\frac{1+\sqrt5}{2}=\frac{42}{w}$$
$$w(1+\sqrt5)=84$$
$$w=\frac{84}{1+\sqrt5}$$
$$w=\frac{84(1-\sqrt5)}{(1+\sqrt5)(1-\sqrt5)}=\frac{84(1-\sqrt5)}{1-5}\approx26$$
