(b) $Maximum\ height\ H = \frac{u^{2}\sin^{2}\theta}{2g}$
$and\ times\ of\ flight\ T = \frac{2usin\theta}{g}$
$or\ \ T^{2} = \frac{4u^{2}\sin^{2}\theta}{g^{2}}$
$\therefore\ \frac{T^{2}}{H} = \frac{8}{g}$
$or\ T = \sqrt{\frac{8H}{g}} = 2\sqrt{\frac{2H}{g}}$