How do you find a angel when you know the sides?

use The Law of Cosines first to calculate one of the angles

then use The Law of Cosines again to find another angle

and finally use angles of a triangle add to 180° to find the last angle.

cos(C) = (a2 + b2 − c2)/2ab

cos(A) = (b2 + c2 − a2)/2bc

cos(B) = (c2 + a2 − b2)/2ca

54.864 meters

step-by-step explanation:

it would open into a square depending on the type of envelope i’m not sure about the second part