The equation of a curve is y= 3x/sqr(1+x) . Given that the equation of the tangent to the curve at the point x=3 is 15x-16y=k, find the value of k.

I know roughly wad to do but after finding dy/dx which is the gradient of the curve as well as the tangent to the curve at the point x=3,

i had problems constructing the equation of the curve(it is too complicated, i dunno how to simplify)....