Okay. so we need to show r=2(p+t)
Refer to the figure rohan.
TOX is congruent to RXQ
So that OX = XR
or OR=2*OX -(1)
now, as SP||OT, PX||OT and also TX||OP
so that OPXT is a parallelogram
Now invoking Parallel Law of vector addition, we can say
but from (1)
Hope it helps.