The triple point is the point at which vapor, gas and liquid all coexist. This point is defined in terms of temperature and pressure. Therefore, the pressure of the gas is equal to the pressure of the liquid is equal to the pressure of the solid (and the same holds true for temperature).
#6.9
"For uranium hexsfluoride the vapor pressure (in Pa) for the solid and the liquid are given by
ln Ps = 29.411 - 5983.5/T
ln Pl = 22.254 - 3479.9/T
Calculate the temperature and pressure of the triple point."
Since Ps = Pl, ln Ps = ln Pl, therefore, 29.411 - 5983.5/T = 22.254 - 3479.9/T.