What is the area of this triangle?
a) a=1/2(y3-y2)(x3-x1)
b) a=1/2(y3-y1)(x3-x1)
c) a=1/2(y2-y1)(x3-x1)

The area of a triangle is:

A=hb/2 where h=height and b=base

We can let the "height" be the horizontal magnitude, (x3-x1)

And the base would then be the vertical magnitude, (y2-y1)

So the area is:

A=(1/2)(y2-y1)(x3-x1)

1.17+3.99a. you combine the terms with a to create 3.99 and the 1.17 stays because it has no like terms or a variable