$3.$ Two point$-$sources are creating circular waves in a pool. You have a little remote$-$controlled motor boat and gently move it to a location where it does not move up or down. The boat is $13\mathrm{.}7$ m from one source and $15\mathrm{.}2$ m from the other source. Very slowly, you now move the boat across a section where it experiences lots of wave action, and settle it into the next quiet spot. At this time, the boat is $12\mathrm{.}1$ m from one source and $13\mathrm{.}0$ m from the other source. Determine the wavelength of the water waves. $(3$ marks$)$