Differentiation

1. given x^2-4xy-2y^2=10
a)find dy/dx
Ans: (x-2y)/2(x+y)
b)find the coordinates of the points on the curve at which the slope is 2.
Ans: (-2,1),(2,-1)
2. if the tangents to the curve y= x^3 +(k-1)x^2 +12x+7 do not parallel to the x-axis, find the range of values of k.
Ans: -5