Please help. I don’t understand how to do this

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