First we can divide this figure into 2 parts, a rectangle down and a quadrilateral up.
For the first rectangle down the area is 11 x 20= 220 m^2.
For the remaining part we have on side which is equal 10 meters and the other part we don’t know it, but if you take the big side on the left and remove from it the part on the right u get the other measurement that you need. So you get:
21 - 11= 10 meters. So now we know that the remaining part is a square because we have all the side that are 10 meters, then we do 10x10= 100m ^2.
Then finally we add both areas for the total area: 220+100= 320m ^2 .