How can you find the area of a polygon that is not for which you know an area formula

See below.

Step-by-step explanation:

If it is a regular polygon then you can look up the formula or break it into shapes you do know the formula.

If it is irregular or a composite figure, break it into regular polygons which do have formulas to find the area. Here is an example:

Example:

Find the area of the composite figure by splitting the figure into two shapes - two rectangles.

One rectangle is 2 x 9 = 18.

The other rectangle is 2 x 4 = 8.

Together the area is 18 + 8 = 26.