We have divided by two this is because you know the are of a rectangle is base + height but a triangle is like cut rectangle into two half pieces of you look carefully than to make any traingle a rectangle you will have to take another on opposite side so that's why we divide it by two
On the coordinate plane it would be a number on the top left and point at a bottom number, the top left is the first number and on the bottom is the last number. Hope that helped!
