Trapezoidal area = (upper bottom+lower bottom) * height divided by 2.

The total length is 40 meters.

The height is on the chart.

Yes 10 meter

Then its (upper bottom+lower bottom) is equal to the total length minus the height, that is, 40m- 10m=30m.

So the area S=30* 10/2= 150 square meters.

Understand?