Area With a Sine

Area is half the base times the height. Without a right angle the height is not given, but a sine finds it. Take sides a and b with the angle C between them, and lay b along the base. The height is the side opposite C in a right triangle with hypotenuse a, so it is $a\cdot \sin\left(C\right)$.

$$A = \frac{1}{2}\cdot a\cdot b\cdot \sin\left(C\right)$$

Sides 4 and 9 with 30° between them: $A = \frac{1}{2}\cdot 4\cdot 9\cdot \sin\left(30^{\circ}\right)$, which is 18 × 1/2 = 9 square units. The angle must be the one between the two sides you multiply. On the board, tap the angle, both sides and the area label, then pick Area with a sine.