There are two answers: one if you take the direct distance between the two locations and the other if you walk along the streets.
The direct answer: use the distance formua: d = sqrt( (x2 - x1)2 + (y2 - y1)2 )
with (x1, y1) = (17, 21) ---> x1 = 17 and y1 = 21
and (x2, y2) = (28, 13) ---> x2 = 28 and y2 = 13
---> d = sqrt( (28 - 17)2 + (13 - 21)2 ) ---> d = sqrt( 112 + (-8)2 ) ---> d = sqrt( 121 + 64 ) = sqrt(185)
which is a distance of about 13.6 blocks
The other answer by walking along the sidewalks: going from 17 to 28 = 28 - 17 = 11 blocks
and going from 21 to 13 = 21 - 13 = 8 blocks
for a total of 11 + 8 = 19 blocks.