Question 568948
The letter m is often used for slope of a line, and that’s what I bet the M in your problem stands for.
The slope of the line connecting  ( n , 11/2 )  and  ( - 1/4 , 0 ) is the difference in y-coordinates divided by the difference in their x-coordinates:
Slope=m={{{(11/2-0)/(n-(-1/4))=(11/2)/(n+1/4)=(11/2)/((4n+1)/4)=(11/2)(4/(4n+1))=11*2/(4n+1)=22/(4n+1)}}}
{{{-22=22/(4n+1)}}} --> {{{4n+1=22/(-22)}}} --> {{{4n+1=-1}}} --> {{{4n=-2}}} --> {{{n=-1/2}}}
The slope of the line connecting  ( 4 , n ) and  ( 1 , a-b) is
Slope={{{(n-(a-b))/(4-1)=(n-a+b)/3}}}
{{{(n-a+b)/3=2b/3}}} --> {{{n-a+b=2b}}} --> {{{n=a-b}}}