If it is a scalene triangle, and you know only the length of the sides, you
can use Heron's (or Hero's) formula:

Suppose the sides are a, b, and c.

Then find the perimeter, P = a + b + c

Then find the semi-perimeter, s, by taking 1/2 of the perimeter P.
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Then Area = Ös(s - a)(s - b)(s - c)

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Whereas a triangle is a rigid, (that is, only the three sides need be
known to determine the area), a trapezium is not.

So if it is a trapezium, or any quadrilateral other than a rectangle
(whether any two sides are parallel or not) then you must measure one
of the diagonals in order to calculate the area.  The diagonal will
then divide the quadrilateral into two triangles, each of whose area
can be calculated using Heron's formula above.  Then the two areas can
be added to give the area of the quadrilateral.

Edwin
AnlytcPhil@aol.com