The area of a parallelogram  is the product of its base measure by its height measure.


The base has the measure of  33+12 = 45 cm.


All you need to find is the measure of the height drawn to the base.


Let ABCD be your parallelogram with the longer side AB of 29+12 = 45 cm long.

Draw the height (the perpendicular) to the side AB from the vertex C.


You will get a right angled triangle with the hypotenuse of 29 cm and one leg of 33-12 = 21 cm.  Hence, its other leg is  = 20 cm.


This other leg is the height of the parallelogram.


Now the area of the parallelogram is  45*20 = 900 cm^2.