Find the area of the following polygons ? how to i solve this

The area of a triangle is found by multiplying the base by the height. Problem is, that in this example with 3 different sides, you can't easily find out the height. 

To the rescue comes Heron's formula:



and



So s = (25+26+21)/2 = 36

and Area = √36·11·10·15 ≈ 243.7