Height of blue triangle = 6 cm so we know that the blue triangle’s dimensionscare twice the red triangle’s.
Base = 5 x 2 = 10 cm
Therefore area of blue triangle
= 1/2 x 6 x 10
= 30 cm2
Shaded area
= area of big triangle - area of blue triangle
= 1/2 x 9 x 15 - 30
= 67.5 - 30
= 37.5 cm2