Given:
The length of the side a is, 11.
The length of the other side b is, a+4 = 11+4 = 15.
The objective is to find the area of trapezium.
The formula to find the area of trapezium is,
Let's find the height of the trapezium using Pythagoras theorem.
Now, substitute the obtianed values in the formula to find the area of trapezium.
Hence, the area of trapezium is 39 square units.