Question 73752
The three given points all lie on the same line, right? Why?, because, you are told, they all are solutions to the same linear equation.
So, we can start by finding the slope of the line, using the first two of the three points.
{{{m = (y[2]-y[1])/(x[2]-x[1])}}} Substitute (0,1) and (4,9).
{{{m = (9-1)/(4-0)}}} 
{{{m = 8/4}}}
{{{m = 2}}} This is the slope of the line. The third point also lies on this line so we can use it, along with one of the other two points, in the slope formula, remembering that m = 2.
{{{2 = (N-9)/(3-4)}}}
{{{2 = (N-9)/-1}}} Multiply both sides by -1.
{{{-2 = N-9}}} Add 9 to both sides.
{{{7 = N}}} or {{{N = 7}}}