Astronauts visiting Planet X have a 2.5 m-long string whose mass is 5.1 g. They tie the string to a support, stretch it horizontally over a pulley 1.5 m away, and hang a 1.8 kg mass on the free end. Then the astronauts begin to excite standing waves on the string. Their data show that standing waves exist at frequencies of 64 Hz and 80 Hz, but at no frequencies in between. What is the value of g, the free-fall acceleration, on Planet X (in m/s^2)?