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find the equations of the two lines, and solve them simultaneously for the intersection

the equation of a line is:

y = a + bx where b is the slope

 

given two points on the line at (x1,y1) and (x2,y2), the slope b can be determined by the expression:

b = (y1 - y2) / (x1 - x2)

 

the value of a can then be found by solving the equation using either point

Done