For a positive number $c$, there is a hyperbola with foci $\mathrm{F}(c, 0)$ and $\mathrm{F'}(-c, 0)$ and major axis length 6. Two distinct points $\mathrm{P}$ and $\mathrm{Q}$ on this hyperbola satisfy the following conditions. Find the sum of all values of $c$. [4 points]\\
(가) Point P is in the first quadrant, and point Q is on line $\mathrm{PF'}$.\\
(나) Triangle $\mathrm{PF'F}$ is isosceles.\\
(다) The perimeter of triangle PQF is 28.