Okay so A and B are the midpoints of PQ and that means the midpoints will divide the sides into equal halves.consider triangle PAB, Here  PA=1/2 of 10 and PB=1/2 of 26.therefor PA and PB will be equal to 5 and 13 you can find the base of the triangle i.e AB using Pythagoras theorem. square of 13 - square of 5=144.root of 144 is 12 so AB=12 cm.
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