Write an equation in standard form of the line that passes through (3,-7) and (9,-2)

Slope = (-2 + 7)/(9 - 3) = 5/6

y = mx + b
-7 = 5/6(3) + b
b = -7 - 5/2
b = - 19/2

equation
y = 5/6x - 19/2

standard form
5/6x - y = 19/2 or 5x - 6y = 57

1) Given
2) Adding segments AB and BC together gives AC (Addition postulate, not too sure about the terminology) 
3) Segment Addition postulate 
4) Segment Addition postulate(Knowing that AC=BD, the distance between BC and CD add up to BD, shown in #3. AB and BC add together to give AC) 
5) Subtraction property of Equality 

Hope I helped :)