Vector [tex]PQ=(7-5, 3-4)=(2, -1)[/tex]. [tex]4RS=4 \cdot (4-8, 1-6) = 4 \cdot (-4, -5) = (-16, -20)[/tex]. So the sum is [tex](-14, -21)[/tex].
For the second, the magnitude is the square root of the sum of the squares of the components. This is equal to [tex]\sqrt{(-14)^2+(-21)^2} = \sqrt{637}=7\sqrt{13}[/tex].