We know that the library is located in te point (4,-1) and the grocery store in the point (-8,6). The distance between points is given by
[tex]d(P,Q)=\sqrt[]{(x_2-x_1)^2+(y_2-y_1)^2}[/tex]In our case, let P be (4,-1) and Q be (-8,6); then
[tex]\begin{gathered} d(P,Q)=\sqrt[]{(-8-4)^2+(6-(-1))^2} \\ =\sqrt[]{(-12)^2+(7)^2} \\ =\sqrt[]{144+49} \\ =\sqrt[]{193} \\ =13.89 \end{gathered}[/tex]So the distance between the point P and Q is 13.89; but we have to remember that each unit represents two blocks then we have to multiply our result by 2.
The distance between the grocery store to the library is 27.78 blocks or, rounding up, 28 blocks.