The line is positive because it is going upwards. The y-intercept is positive 3. It can be found by seeing where the line intercepts the y-axis. The slope of the line is 2 and it can be found by using "rise over run."
(I saw the image on another question you had asked. Have a nice day!)
BC/DC = 8/3.2 = 2.5
AB/BD = 12.5/5 = 2.5
AC/BC = 20/8 = 2.5
Since the results are all 2.5, this means that the triangles are similar triangles.
The justification you would use is the SSS similarity theorem.
SSS = side side side
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2+(1/2) +34- (1/10)
2.5 + 34 - .1
36.5-.1
36.4
