can someone please expalin this question in detail with each step and where they are getting the number from:

Archaeologists are excavating a square plot ABCD measuring 50 metres by JU metres. The plot is divided into four sections of equal width as shown in the diagram below. At the end of the first phase of digging, the shaded area has been excavated. Measurements made to the edges of the excavated area are also shown on the diagram. D NOT TO SCALE 19 m 22 m 18 m 21 m 132 m A B 50 m (a) Use the trapezoidal rule with each of the measurements given to estimate the shaded 2 area