i. application of Pythagoras ...
$|MB|=\sqrt{30^2-10^2} = \sqrt{800} = 20\sqrt{2} \, cm$ ... the length of MB, the height of triangle ABC, is in "surd" form.
Now that you have that necessary piece of information, give the remainder of these questions a go.
base area = $\dfrac{b \cdot h}{2} = \dfrac{20 \cdot 20\sqrt{2}}{2} = 200\sqrt{2} \, \, cm^2$
recommend you learn some arithmetic with radicals ...
radical+arithmetic