# Thread: Slope of tangent #3

1. ## Slope of tangent #3

find the points on the graph of y=3x/(x-4), where the slope of the tangent is -12/25

my work: (but I can't seem to find x)

i took derivative of f(x)
y'=[3(x-4)-3x(1)]/[x-4]^2
y'=-4/(x-4)^2

i let y' = -12/25
and tried to solve for x.. no luck -help?

2. $y'=-\frac{12}{(x-4)^2}$

$-\frac{12}{(x-4)^2}=-\frac{12}{25}\Rightarrow (x-4)^2=25\Rightarrow x-4=\pm 5\Rightarrow x_1=9, \ x_2=-1$