You take your ruler and put it along the edge you want to merasure, line the begining of the scale (0) up with the corner and read off the length at the other corner.
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It is the base.
Divide the length of one side by the length of an adjacent side.
It is usually called the "base".
A parallelogram is just a rectangle leaned to one side
You cannot. With only the base given (a parallelogram) you don't even know what the shape is: it could be a parallelepiped, or a parallelogram based pyramid or one of several other shapes.