Let the sides of the triangle ABC be:

a = 26 cm

b = 30 cm

And

c = 28 cm

Let us find out the value of s

We know that,

s =

=

=

= 42 cm

We know that,

Area of triangle ABC =

=

=

=

=

= 14 × 3 × 4 × 2

= 336 cm²

We know that,

In a parallelogram, the diagonal divides the parallelogram in two equal area

Therefore,

Area of quadrilateral ABCD = Area of triangle ABC + Area of triangle ACD

= Area of triangle ABC × 2

= 336 × 2

= 672 cm²