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?