2. The equation of a horizontal line is of the form y=k.

If it is a tangent to the curve, the derivative must be zero,

since the derivative formulates the slope of the tangent,

hence for what values of x or y is the numerator of the derivative zero?

3. This is the line y=-x.

The slope of a line is it's derivative.

Hence the derivative is -1, and you can substitute y=-x into the derivative to solve for x, and thus y.