
ΔABC has vertex A(1,2)
Given that midpoint of AC is D and mid point of AB is E
Also,
Equation of medium BD is x + y = 5
Equation of medium CE is x = 4
Let coordinate of B(p, q) and C(r, s)

And point B(p, q) lies on the line BD [x + y =4]
∴ p + q = 4
⇒ q = −3
∴ B ≡ (7, −3)
Point C(r, s) lies on CE [x = 4]
∴ r = 4

∴ Coordinates of B & C are (7,−3) & (4,3).