Solve Perpendicular Lines: A(1,2), B(4,0), C(k,2), D(1,-3)

[bacon]

Determine a constant real number k such that the lines AB and CD are perpendicular.
A(1,2), B(4,0), C(k,2), D(1,-3). (answer given is k=-3/2)

If two lines are perpendicular the product of their slopes is -1.

The slope of AB is \frac{0-2}{4-1} = \frac{-2}{3}

The slope of CD is \frac{-3-2}{1-k} = \frac{-5}{1-k}
I set the product of these slopes equal to -1 and solve for k.

\frac{-2}{3} x \frac{-5}{1-k}=-1
10=3(k-1)
10=3k-3
13=3k
13/3=k
This is not the answer given and I am not seeing my error. Any help would be appreciated.
Latex question. The = sign and x symbol don't line up well with the rest of the equations in the first half of my post. How can I correct that?

 

[rocomath]

Your k value is correct ...

\frac{0-2}{4-1}=\frac{-2}{3}

Check my LaTeX ...

 

[bacon]

Thanks for the help.

 

[BrendanH]

Verified using an alternate method (direction cosines and perpendicularity) to confirm that your answer is correct.

 

[mubashirmansoor]

You are 100% correct, reporting the textbook error might be helpful for the rest using the same book...