First, look at the whole square including the dotted lines. We can add up the lines on the right to now vertically the length is 9 and we already see horizontally it’s also 9. So the whole square is 81units^2. Now, we need to subtract the two triangles from this.
All we need to use is the simple triangle area formula. This is A=.5bh. A is area, b is base, h is height. On the left triangle, 4 units is h and 3 units is b. Plug in for A=.5(4)(3). This comes out to 6units^2.
For the second triangle, do the same thing. Here is the height is 5 units and the base is 4 units. This means A=.5(5)(4), giving us an area of 10units^2.
You can add up the two triangles and know that out of the rectangle the triangles take up 16units^2.
All you need to do now is so 81-16, and your answer is 65units^2. Hope this helped!