The sqrt of 19 is between 4.3 and 4.4. The sqrt of 18 is between 4.2 and 4.3. The sqrt of 22 is between 4.6 and 4.7. And finally, the sqrt of 21 is between 4.5 and 4.6.
Call the intersection of 2 diagonal lines as I(a;b) => I is the midpoint of the diagonal PR <=> a= (-2+2)/2. And. b=(3+6)/2 <=> a=0. And. b= 9/2 => I(0;9/2)