How do i find this out?

8

Explanation:

A good way to find the area of a figure with diagonal lines is to find the area of the section that has full squares and then add the other parts together and find the area of that. As you can see in the polygon, there are four units in the middle that don't get cut off by the diagonal line. The area of that part of the figure is 1*4 = 4. Now, if you take out that section, and move the other parts together, you can find their area and then add it to 4. The other parts of the polygons are triangles, and when you move them together, they form the same shape as the middle section did, and have an area of 4. The area of the entire figure is the area of one section plus the area of the other. 4 + 4 = 8

I hope that makes sense :)