f(-1/3x) is a transformation that reflects the function over the y-axis and dilates it horizontally by a factor of 3. When (5, 4) is reflected over the y-axis, it becomes (-5, 4). When points are then moved horizontally by a dilation factor of 3, the x-value becomes 3×(-5) = -15.
The point corresponding to (5, 4) will be (-15, 4).