We have that enlarging the rectangle in a computer setting keeps the ratio of the lines constant. Hence, height over length is the same. For the first rectangle this is 6/13. For the second one, if x is the new height, this is x/19.5.
Hence x/19.5=6/13. Multiplying both parts by 19.5 we have that x=19.5*6/13=9. Thus, the new height is 6 cm.