Problem
Draw the crease FC; corner B is square, so BFC is a right triangle with FC as its longest side.
Drawing the picture turns the folding story into a plain right triangle, which is much easier to reason about than the folded paper itself.
6.G.A.1Draw A DiagramWith BF = 9 and FC = 15, the missing side BC is 12, the familiar 9-12-15 right triangle.
The square corner of the rectangle guarantees a right triangle, so the three sides must fit together as 9, 12, 15.
8.G.B.7Identify SubproblemsThe fold lays the bottom piece onto FE without stretching it, so the width is 9 + 12 = 21 cm.
Imagining the paper folding back flat shows the folded edge is just the original bottom piece in a new place, so it keeps its length.
6.G.A.1Create A Physical RepresentationThe folded-over edge keeps exactly the length it had before the fold.
Folding lays one part of the paper onto another place without stretching or tearing it, so every length is carried across unchanged.
The full width can then be read as the folded piece together with the piece that stayed where it was, with nothing missing between them.
The rectangle is 21 cm wide and 12 cm tall, so its area is length times width: 21 x 12 = 252.
Area of a rectangle is just its two side lengths multiplied together.
4.MD.A.3Identify SubproblemsA fold never stretches the paper, so a folded edge keeps its length-use that, plus the 9-12-15 right triangle, to rebuild the whole sheet.