It's the same deal with this question as it was with the other one you posted. We will use Pythagoras' Theorem to find the length of the other side, and then the half-base*height formula for the area of the envelope (which I assume is the "rectangle" mentioned). I hope you realize why we use the two shorter sides to find the area. If not, it's because they are at right angles to each other, so if we places one of the shorter sides on the ground, the other short side would be the vertical height. Anyway, here goes:

Using a^2=b^2+c^2 (Pythagoras' Theorem: a -hypotenuse, b,c-other two sides)

=> a^2-b^2 = c^2

using a=35 cm, b=21 cm, we get

35^2 -21^2 = c^2

784 = c^2

squareroot(784) = c

=> c = 28

For area:

A = 0.5*base*height

= 0.5*21*28

= 294 cm^2