please explain this well i don't understand (55 points)

Answer:
B
Step-by-step explanation:
the area of a triangle is
[tex]A=\frac{1}{2}(h)(b)[/tex]
so if we take as
[tex]from\:(x_1,y_1)\rightarrow(x_2,y_1)[/tex]
so the base is
[tex]b=x_2-x_1[/tex]
so the height is
[tex]from\:b\rightarrow(0,0)[/tex]
the distance is
[tex]h=y_1-0[/tex]
so the area is
[tex]A=\frac{1}{2}(y_1+0)(x_2-x_1)\\\\=\frac{1}{2}(y_1)(x_2-x_1)[/tex]
so the answer is B