The slope of CD = (9-4) / (-5-5) = 5/-10 = -1/2
The slope of AB must also be -1/2
Slope of AB = (1-7) / (x - -3) = -6 / (x+3)
For AB to equal -1/2, the denominator needs to be twice as much as the numerator, so set up an equation:
2(x+3) = 12
Now solve for x:
2x +6 = 12
2x = 18
x = 18/2
x = 9
