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Answer:
A = (y2 -y1)(x3 -x1)/2
Step-by-step explanation:
It is convenient to use the vertical line as the base. Its length will be the difference of y-coordinates: (y2 -y1)
Then the altitude is the perpendicular distance from that line to (x3, y3). That distance will be the difference of the x-coordinates: (x3 -x1).
The area is given by the usual formula for the area of a triangle:
A = 1/2bh
A = 1/2(y2 -y1)(x3 -x1)
