Hi!
Let's separate this shape into three sections to make it easier to find the area. (see image)
Let's start with the green shape.
The base of the green shape is 20. The height is 6.
20 x 6 = 120
Now the blue shape.
The base of the blue shape is 20, the height is (9 + 6) 15.
20 x 15 = 300
Now the red shape.
The base of the red shape is (20 - 10) 10, the height is 5.
10 x 5 = 50
Add all of the areas together.
120 + 300 + 50 = 470
The answer is 470
Hope this helps! :)