The answer depends on the what characteristic of the pyramid you want the formula for: its surface area, its volume or something else.
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A "formula" is some mathematical rule for calculating something you wish to know. So "the formula of the rectangle-base pyramid" will be some expression which tells you something that you want to know about it. Beyond that there is no way anyone can tell you, since you do not say what feature of the pyramid you are thinking about (height, volume, surface area, weight...).
The derivation of the formula of pyramid can be gained easily based on the formula for a triangular prism. A pyramid is like two prisms joined together.
A rectangle based pyramid or just a random 2-d sketch.
A pentagonal pyramid, A rectangle based dipyramid, A triangular prism.
It is a rectangle.