Here is a cube of side $a$:

We find the net of a cube by unfolding it, like this:

This animation shows the cube being completely unfolded:

The surface area of the cube is the total area of all its faces. Assuming the side of the cube has length $a$, each face has an area of $a^2$:

Since there are 6 faces, the total surface area is:

$$ area = 6 a^2 $$

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