Explain why points M, N and L cannot lie on the same straight line if MN = 8 in, ML = 5 in, NL = 6 in.

Point L must be between points M and N, otherwise any of the segments ML or NL must be greater than 8 dm.
But in this case, both segments are less than 8 dm.
On the other hand, then the sum of the segments ML + NL should be equal to 8 dm.
But the sum ML + NL = 5 + 6 = 11 dm.
As you can see, none of the conditions is satisfied, which means that the points M, N and L cannot lie on one straight line.