If the tangent line at a point (x, y) on the curve y = f(x) is parallel to x-axis, then write the value of 
Given curve y = f(x) has a point (x, y) which is parallel to x-axis.
We know that the slope of the x-axis is 0.
∵ the point lies on the given curve
∴ Slope of the tangent 
1