jmc

For the roots to be real and rational, the discriminant must be a perfect square. Therefore, $(-7{)}^2-4\cdot 1\cdot c=49-4c$ must be a perfect square. The only positive perfect squares less than 49 are $1$, $4$, $9$, $16$, $25$, and $36$. The perfect squares that give a integer value of $c$ are $1$, $9$, and $25$. Thus, we have the equations $49-4c=1$, $49-4c=9$, and $49-4c=25$. Solving, we get that the positive integer values of c are $\boxed{12,10,6}$.