Answer: PQ = 18 unit.
Explanation: Since, according to question,
.
Therefore, by the property of similar triangles the ratio of corresponding sides must be equal.
Here, The right angle are at A in
and at Q in
respectively.
Moreover,
is congruent to
and
is congruent to
.
Therefore, AB, BC and AC are corresponding to sides PQ, QR and PR respectively.
Thus, we can write, 
⇒
⇒
( because, here, AB= 6, BC=8 and QR=24)⇒ PQ=18