The area of a triangle is found using the formula 1/2 x base x height.
The larger triangle has a base of 20cm and a height of 18cm.
The area would be 1/2 x 20 x 18 = 180 square cm.
The white triangle has a base of 20cm and a height of 8cm.
The area would be 1/2 x 20 x 8 = 80 square cm.
The area of the shaded portion would be the larger triangle minus the smaller triangle:
180 - 80 = 100 square cm.