Let

d = the distance between P and Q

d^2 = (5 - a)^2 + (2a - 3)^2

d^2 > 26^2

(5 - a)^2 + (2a - 3)^2 > 26^2

5a^2 - 22a - 642 > 0

The roots of the quadratic equation 

5a^2 - 22a - 642 = 0

........

click here to see the equation solved for a

........

a1 = 13.74

a2 = - 9.34

For a in the interval (- ~, -9. 34) U (13.74, +~) the distance is greater than 26.