Explanation:
y = -f(x)
so, everything is mirrored around the x-axis.
what was below the x-axis, is now above the x-axis, what was above is now below. and what was on the x-axis, is still on the x-axis.
pick the corner points of f(x), their x-values stay the same, their y-values flip the sign. the points on the x-axis (y = 0) stay the same of course.
and then connect the points, so that the curve is upside-down to the original.