Question 1199552
.


                                Part   (a) 



If you draw the given points in a coordinate plane, you will see


(even with unarmed eye)  that one pair of sides is horizontal   (y = 3   and   y = 8)  of the length of  5  units, 


while the height of the parallelogram (the distance between its parallel horizontal sides)  is  8 - 3 = 5  units.


It gives the area   5*5 = 25  square units.



In part  (b),  the same reasoning does work,  allowing you to get the answer  MENTALLY.



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At the beginning of your post you write


        " I've been trying to use the same formula that I use to find 

        the area of a triangle but it is not giving me the correct answer. "



It is not surprising,  because you have no any reason to use 
" the same "   formula for totally different problem.


It is like jumping off a tower upside down into a swimming pool with no water.



In opposite,  it would be surprising if this formula would work.