Multiply the length by the wide. eg a 3cm x 6cm rectangle - 3x6=18sqcm
The answer is in square units, - square feet, metres, mm etc
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You could consider the cross as two intersecting rectangles. Calculate the area of both rectangles and the area of the intersection (overlap). Then area of cross = sum of the areas of the rectangles minus the area of the overlap.
Perimeter: add all sides area: multiply length times width for rectangles
There is no such thing as an "irregular rectangle". To calculate the area of a rectangle - if that's what you mean - you multiply length x width.
You divide the shape into smaller shapes you can calculate, like rectangles and triangles. If the shape is irregular, you have to approximate, for example by dividing it into many narrow rectangles. This technique is called "integration".
An L is two rectangles. Just make sure you only count the place where they intersect once.