Greetings from Brasil...
Let's apply the given formula:
A = (1/2)·B·H
The base of this polygon (in this case, the triangle) is B
B = X² - 2X + 6
The height of this polygon is H and is H
H = X + 4
Applying these values (B and H) in the given formula.....
A = (1/2)·B·H
A = (1/2)·(X² - 2X + 6)·(X + 4)
A = (1/2)·(X³ + 2X² - 2X + 24)
A = (X³/2) + X² - X + 12
OR
A = (X³ + 2X² - 2X + 24)/2
