Find the area of the following composite shape

Explanation

1. The idea behind this shape is that there is a big triangle and a small one (dotted line base).  The small triangle area is subtracted from the big triangle
   • A_shape = A_Δlarge - A_Δsmall
2. Write the rule of triangle area
   • A_triangle = 1/2 bh
3. Calculate small triangle area where b = 12 and h = 5
   • A_Δsmall = 1/2 * 12 * 5 = 30 cm²
4. Calculate large triangle area where b = 12 and h = 18
   • A_Δlarge = 1/2 * 12 * 18 = 108 cm²
5. Calculate the area of the shape
   • A_shape = 108 - 30 = 78 cm²