The x-axis is labelled as radius (r). Since the question is asking you to find the velocity of the fluid, which is labelled on the y-axis, you simply need to move along the x-axis until you reach 3 cm and move upwards until you've reached the line. Once you have reached the line, the answer should be on the y-axis which is 27 cm/s.
*x-axis is along the bottom and y-axis is vertical along the left side.*
