Since PQRS is a parallelogram
PQ||RS (opposite sides of parallelogram are parallel)
And ABRS is also a parallelogram
So, AB||RS (opposite sides of parallelogram are parallel)
Since PQ||RS and AB||RS
We can say that PB||RS
Now, PQRS and ABRS are two parallelograms with the same base RS and between the same parallels PB and RS
Therefore, ar(PQRS)=ar(ABRS)