A4 What is the area of the shaded triangle?
(A) $6 \mathrm{~cm}^{2}$
(B) $9 \mathrm{~cm}^{2}$
(C) $15 \mathrm{~cm}^{2}$
(D) $18 \mathrm{~cm}^{2}$
(E) $30 \mathrm{~cm}^{2}$
A4. The area of a triangle is calculated by dividing the product of a side of the triangle and the height to that side by 2. The area of the shaded triangle in the figure is equal to $\frac{3 \mathrm{~cm} \cdot 6 \mathrm{~cm}}{2}=9 \mathrm{~cm}^{2}$.