In ∆PQR, M is the mid-point of side PQ.
∴ ar (∆PMR) = ar (∆MQR) ... (1) (Median of a triangle divides it into two triangles of equal areas.)
Now, ar (∆PQR) = ar (∆PMR) + ar (∆MQR)
⇒ ar (∆PQR) = 2 × ar (∆PMR) [Using (1)]
⇒ ar (∆PQR) = 2 × 13 [ar (∆PMR) = 13 cm²]
= 26 cm²