John needs a sheet of paper x cm by 15 cm for an origami crocodile. The local origami supply store only sells square sheets of paper.
The lower portion of the image below shows the excess area A of paper that will be left after John cuts out the x cm by 15 cm piece. The excess area, in cm², is given by the equation A = x(x-15). At what lengths x will the excess area be zero? Write all solutions on the same line, separated by a comma.