Semiperimeter of DABC                               
$\mathrm{s}=\frac{26+28+30}{2}=42\mathrm{cm}$
s – a = 42 – 26 = 16 cm
s – b = 42 – 28 = 14 cm                               
s – c = 42 – 30 = 12 cm
Area of $\mathrm{\Delta ABC}=\sqrt{\mathrm{s}(\mathrm{s}–\mathrm{a})(\mathrm{s}–\mathrm{b})(\mathrm{s}–\mathrm{c})}$
$=\sqrt{42\times 16\times 14\times 12}{\mathrm{cm}}^2=\sqrt{14\times 3\times 4\times 4\times 14\times 4\times 3}{\mathrm{cm}}^2$
= 14 × 4 × 3 × 2 cm² = 336 cm²
$\therefore$    Area of parallelogram = Area of triangle [Given]
        h × AB = 336
        h × 28 = 336 cm²
        $\mathrm{h}=\frac{336}{28}=12\mathrm{cm}$