The estimated lengths are 12in. and 5 in. This would be an estimated difference of 7in.
The estimated difference is going to be higher than the actual difference because in rounding 11.7 to 12, you are losing .30in, but in rounding down 5.25 to 5, you are actually gaining .75in.
The ACTUAL difference is 6.45in
Solution: Check the picture I uploaded.
The first one because the slope is 2/1 and y-intercept is -4